As a long time owner of a 70 GT500 (since 1981), I have often thought about why the
69/70 Shelbys received special rear leaf springs. And how do they compare to regular Mustang springs? Does anybody know the answer(s)?
Tom
all fords received different springs. it depended on options such as auto/stick, type of suspension, ac or no ac, ht or convert etc. it had to do with weight distribution as much as handeling. if you can find an original (not updated) ford mpc, you can see all the different combos, weights, color stripes and so on. most were superceeded by a spring or 2 it really didn't matter much about ride height (minimal at best) I tried to raise the back of my 68 gt 350 using 500 convert, ac springs heavyiest listed, didn't change any thing. think of axle weight also, for the purpose of braking. phred
69 shelbys (except for the very early cars) had unique s9ms 4 leaf 2 wrap rear springs then c9zx springs (and they look the same as s9ms) started showing up later in the year . I put them side by side & dont see a difference just engineering # change & both show up on gt350 & 500 4-speed &auto fastback & vert & a/c cars.
true, but the spring rates are different (not by much)
On a similar train of thought, I wonder why only the 1969/70 GT350 was equipped with the 9984 coded rear axle. It wasn't shared with any other mustang or cougar based upon my research, which may be faulty. ???
I think the 69 shelby unique leaf spring had something to do with the new f-60-15 tires that were supposed to be standard on the new 69 shelbys & the fact that previous shelbys (except for 68s) had some kind of rear traction help. The 69 shelby 2 leaf spring wrap never had a wrap on the bottom leaf & it was a straight leaf with no curve (kinda like a slapper bar type) vs regular mustang 3 wraps & the bottom leaf had a wrap. I think the s9ms # change to c9zx # was same spring just a running change like 69 b9 parts started out with kkx#s then went to c9zx#s same parts just changed to a regular ford #.
Quote from: 69 GT350 Vert on November 03, 2018, 11:18:22 AM
On a similar train of thought, I wonder why only the 1969/70 GT350 was equipped with the 9984 coded rear axle. It wasn't shared with any other mustang or cougar based upon my research, which may be faulty. ???
I have at least one 69 M Code Mach I with a 3 speed manual buildsheet, with that same rear end code
Not allot of stripped down Mach I's out there and that may have got in the way of finding other examples
Quote from: J_Speegle on November 03, 2018, 02:47:56 PM
Quote from: 69 GT350 Vert on November 03, 2018, 11:18:22 AM
On a similar train of thought, I wonder why only the 1969/70 GT350 was equipped with the 9984 coded rear axle. It wasn't shared with any other mustang or cougar based upon my research, which may be faulty. ???
I have at least one 69 M Code Mach I with a 3 speed manual buildsheet, with that same rear end code
Not allot of stripped down Mach I's out there and that may have got in the way of finding other examples
Interesting. Do you know if the bottom leaf on that Mach 1 has the S9MS identifier on it or the 5RS68(as on my GT350)?
Pretty sure the S9MS spring is unique to Shelbys as it was installed by AOSmith, not Ford assembly.
It has a lower load rate than most other Mustang rear springs.
Quote from: Coralsnake on November 04, 2018, 12:31:00 PM
Pretty sure the S9MS spring is unique to Shelbys as it was installed by AOSmith, not Ford assembly.
It has a lower load rate than most other Mustang rear springs.
You can be sure it is 69/70 Shelby unique.
Quote from: 69 GT350 Vert on November 04, 2018, 11:59:29 AM
Quote from: J_Speegle on November 03, 2018, 02:47:56 PM
Quote from: 69 GT350 Vert on November 03, 2018, 11:18:22 AM
On a similar train of thought, I wonder why only the 1969/70 GT350 was equipped with the 9984 coded rear axle. It wasn't shared with any other mustang or cougar based upon my research, which may be faulty. ???
I have at least one 69 M Code Mach I with a 3 speed manual buildsheet, with that same rear end code
Not allot of stripped down Mach I's out there and that may have got in the way of finding other examples
Interesting. Do you know if the bottom leaf on that Mach 1 has the S9MS identifier on it or the 5RS68(as on my GT350)?
Are you implying that the M code Mach I also used that same 5RS68 leaf spring?
Quote from: 69 GT350 Vert on November 04, 2018, 11:59:29 AM
Quote from: J_Speegle on November 03, 2018, 02:47:56 PM
Quote from: 69 GT350 Vert on November 03, 2018, 11:18:22 AM
On a similar train of thought, I wonder why only the 1969/70 GT350 was equipped with the 9984 coded rear axle. It wasn't shared with any other mustang or cougar based upon my research, which may be faulty. ???
I have at least one 69 M Code Mach I with a 3 speed manual buildsheet, with that same rear end code
Not allot of stripped down Mach I's out there and that may have got in the way of finding other examples
Interesting. Do you know if the bottom leaf on that Mach 1 has the S9MS identifier on it or the 5RS68(as on my GT350)?
It would not have had the Shelby springs. Rear axle code has nothing to do with rear springs that were assigned and installed in the car. Two different things.
Thanks for the clarification that the springs are not correlated to rear axle code. I didn't know that. I always thought that the rear springs were part of the rear axle assembly.
The axle code chart I read on dead nuts on website said the 9984 code was exclusive to the shelby. Since a Mach 1 was found with the 9984 coded axle, I guess that is not true. Just wondering what is different about the 9984 axle that caused Ford to create a separate code for it. I always thought it was for the leaf springs, but that isn't the case based upon the responses.
Quote from: 69 GT350 Vert on November 04, 2018, 04:45:07 PM
Thanks for the clarification that the springs are not correlated to rear axle code. I didn't know that. I always thought that the rear springs were part of the rear axle assembly.
Ford designed allot of different rearends and to keep track of everything they had to assign them each their own codes. Things like housing differences, axle spline count, carrier used, yoke used, pinion retainer used, axle snubbers or no axle snubber, axle ratio, brake line attachment location and many many more details all come into play and by changing one a different coded axle assembly was needed. Sterling plant assembled these as a unit, marked and labeled each then sent them off to the car assembly plants based on need and orders. Springs and all the attaching hardware came from other different suppliers to the plant. If the rearend and leaf springs had been a single subassembly there would be no reason to list the axle and the rear springs separately on the buildsheet
Just looking at 69 Mustangs there were at least 19 RPO combinations/different axle codes possible. Each different from one another so when you ask "Just wondering what is different about the 9984 axle that caused Ford to create a separate code for it. " you would have to choose which other of the 19 you wanted to compare it too.
Would Eaton have the specifications for these springs?
I found this information in a General Spring application chart. It shows the aftermarket part number of 42-503 as the correct spring for a 69 Shelby.
I don't know if this information is accurate or not.
Quote from: Dennis Y on June 12, 2022, 03:34:28 PM
Would Eaton have the specifications for these springs?
A few years back when I discussed it with them they argued that they did not exist. :o I even sent them a photo but no comment. I guess the know it all I was corresponding with did not want to admit he might not know it all. ;) FYI although they do have Ford engineering drawings from Ford they are NOT the same EATON that made springs for Ford like they sometimes lead people to believe. That Eaton is or was part of a huge industrial conglomerate. They are a different company that happens by coincidence or design to have the same name.
Quote from: 69 GT350 Vert on June 12, 2022, 03:52:16 PM
I found this information in a General Spring application chart. It shows the aftermarket part number of 42-503 as the correct spring for a 69 Shelby.
I don't know if this information is accurate or not.
That would be a replacement substitute no better then the universal replacement that Ford sold for decades to fit just about all Mustangs 65-70. I have bought ones from Virginia Classic Mustang that look the same but are a little less then Eaton plus Brant is a forum contributor. Basically you get a generic replacement regardless of who you buy from. Eaton covers their springs in a thick black paint which for a driver is fine because they will be well protected from the elements but get ready for a major amount of work trying to get all that paint off if trying to make them look close to original. Virginia's still have paint but not near as thick. Regardless of where you get a replacement the leaf's will not all be the proper length and the bottom leaf will look different . At least compared to a assemblyline leaf . ;)
Quote from: Dennis Y on June 12, 2022, 03:34:28 PM
Would Eaton have the specifications for these springs?
Not sure that they were even the supplier of the single leaf - early or later versions
Quote from: J_Speegle on June 12, 2022, 08:08:49 PM
Quote from: Dennis Y on June 12, 2022, 03:34:28 PM
Would Eaton have the specifications for these springs?
Not sure that they were even the supplier of the single leaf - early or later versions
Jeff the aftermarket spring Eaton is NOT the OEM and not the same EATON that made springs for Ford like they sometimes lead people to believe. That Eaton is or was part of a huge industrial conglomerate that is no more. The after market Eaton currently selling replacement springs are a different company that happens by coincidence or design to have the same name.
Quote from: Bob Gaines on June 12, 2022, 08:28:52 PM
Jeff the aftermarket spring Eaton is NOT the OEM and not the same EATON that made springs for Ford like they sometimes lead people to believe. That Eaton is or was part of a huge industrial conglomerate that is no more. The after market Eaton currently selling replacement springs are a different company that happens by coincidence or design to have the same name.
Understood that :) Just a comment about if they were originally
Quote from: papa scoops on November 02, 2018, 02:05:43 PM
all fords received different springs. it depended on options such as auto/stick, type of suspension, ac or no ac, ht or convert etc. it had to do with weight distribution as much as handeling. if you can find an original (not updated) ford mpc, you can see all the different combos, weights, color stripes and so on.
phred
One of the Boss members had a digital copy of the original MPC made if anyone is interested?
Roy
Quote from: TA Coupe on June 13, 2022, 12:44:50 AM
One of the Boss members had a digital copy of the original MPC made if anyone is interested?
Roy
One of the 69 printings ;)
Quote from: Dennis Y on June 12, 2022, 03:34:28 PM
Would Eaton have the specifications for these springs?
If that were the case I bet the results would not be even similar to the specifications. My experiences with that supplier have been nothing short of dreadful. They are not an original supplier and if they do own any specifications they sure do not know how to use them.
Quote from: J_Speegle on June 13, 2022, 01:09:38 AM
Quote from: TA Coupe on June 13, 2022, 12:44:50 AM
One of the Boss members had a digital copy of the original MPC made if anyone is interested?
Roy
One of the 69 printings ;)
Link to the post about the MPC that I was referring to which you were part of.
It covered up to 1970 if I remember correctly.
https://www.boss302.com/smf/index.php?topic=86274.0
Roy
Quote from: TA Coupe on June 13, 2022, 11:29:05 AM
Link to the post about the MPC that I was referring to which you were part of.
It covered up to 1970 if I remember correctly.
https://www.boss302.com/smf/index.php?topic=86274.0
Roy
Seems to be a mix of two printings but yes covers all or some of 70 depending.
Illustrations is an Aug 70 printing where the text section is an Oct 69 printing .
Good to have compared to the last printing IMHO
Quote from: Bob Gaines on June 12, 2022, 04:08:22 PM
Quote from: 69 GT350 Vert on June 12, 2022, 03:52:16 PM
I found this information in a General Spring application chart. It shows the aftermarket part number of 42-503 as the correct spring for a 69 Shelby.
I don't know if this information is accurate or not.
That would be a replacement substitute no better then the universal replacement that Ford sold for decades to fit just about all Mustangs 65-70. I have bought ones from Virginia Classic Mustang that look the same but are a little less then Eaton plus Brant is a forum contributor. Basically you get a generic replacement regardless of who you buy from. Eaton covers their springs in a thick black paint which for a driver is fine because they will be well protected from the elements but get ready for a major amount of work trying to get all that paint off if trying to make them look close to original. Virginia's still have paint but not near as thick. Regardless of where you get a replacement the leaf's will not all be the proper length and the bottom leaf will look different . At least compared to a assemblyline leaf . ;)
Based on info in an older thread, the VA Mustang leaf spring is industry replacement number 42-437, which has free arch of 6.25 inches. The 42-503 spring listed by General Spring for the 69 Shelby has free arch of 3.75 inches, which would provide a lower ride height. It is likely the spring rates are also different. Fortunately, my early original leaf springs stamped 5RS68 are not sagging. However, the original front and rear bushings are cracked. I should pull them off and change them, but the car rides nice as is so I haven't messed with them.
Quote from: Bob Gaines on June 12, 2022, 04:08:22 PM
Quote from: 69 GT350 Vert on June 12, 2022, 03:52:16 PM
I found this information in a General Spring application chart. It shows the aftermarket part number of 42-503 as the correct spring for a 69 Shelby.
I don't know if this information is accurate or not.
That would be a replacement substitute no better then the universal replacement that Ford sold for decades to fit just about all Mustangs 65-70. I have bought ones from Virginia Classic Mustang that look the same but are a little less then Eaton plus Brant is a forum contributor. Basically you get a generic replacement regardless of who you buy from. Eaton covers their springs in a thick black paint which for a driver is fine because they will be well protected from the elements but get ready for a major amount of work trying to get all that paint off if trying to make them look close to original. Virginia's still have paint but not near as thick. Regardless of where you get a replacement the leaf's will not all be the proper length and the bottom leaf will look different . At least compared to a assemblyline leaf . ;)
I bought my Eaton springs from NPD in the late 90s. I thought they were dark and covered with cosmoline. When the car was done it matched the height in my manuals for the comp suspension. I had an issue with my "great Mi" roads even back in the early 2000s of hitting bumps or pot holes and one of my rear 245-60s hitting the wheel lip. I took my springs off and took them to the Eaton small plant in Detroit(across from the old Tiger Stadium on Mich ave for locals) one morning. I drove around Detroit while they re-arced them. I wanted a 1/2 inch higher. I got 3/8-7/16 close enough. I like my ride height now. Now ironically while changing out a sending unit yesterday on my Vert I was looking at my springs and they do not have a heavy coat of black paint on then. Might be a newer thing now? I'll investigate further today as my car is still high on jack stands. I eventually want to put my originals back on or use the lower leaf. in the pic below I have my original front coil springs on the car also.
Quote from: shelbymann1970 on June 14, 2022, 05:28:54 AM
Quote from: Bob Gaines on June 12, 2022, 04:08:22 PM
Quote from: 69 GT350 Vert on June 12, 2022, 03:52:16 PM
I found this information in a General Spring application chart. It shows the aftermarket part number of 42-503 as the correct spring for a 69 Shelby.
I don't know if this information is accurate or not.
That would be a replacement substitute no better then the universal replacement that Ford sold for decades to fit just about all Mustangs 65-70. I have bought ones from Virginia Classic Mustang that look the same but are a little less then Eaton plus Brant is a forum contributor. Basically you get a generic replacement regardless of who you buy from. Eaton covers their springs in a thick black paint which for a driver is fine because they will be well protected from the elements but get ready for a major amount of work trying to get all that paint off if trying to make them look close to original. Virginia's still have paint but not near as thick. Regardless of where you get a replacement the leaf's will not all be the proper length and the bottom leaf will look different . At least compared to a assemblyline leaf . ;)
I bought my Eaton springs from NPD in the late 90s. I thought they were dark and covered with cosmoline. When the car was done it matched the height in my manuals for the comp suspension. I had an issue with my "great Mi" roads even back in the early 2000s of hitting bumps or pot holes and one of my rear 245-60s hitting the wheel lip. I took my springs off and took them to the Eaton small plant in Detroit(across from the old Tiger Stadium on Mich ave for locals) one morning. I drove around Detroit while they re-arced them. I wanted a 1/2 inch higher. I got 3/8-7/16 close enough. I like my ride height now. Now ironically while changing out a sending unit yesterday on my Vert I was looking at my springs and they do not have a heavy coat of black paint on then. Might be a newer thing now? I'll investigate further today as my car is still high on jack stands. I eventually want to put my originals back on or use the lower leaf. in the pic below I have my original front coil springs on the car also.
Decades ago the Eaton Co.in question did not paint them. Sometime later they started to paint them but would leave them unpainted on special request. For the past ten years or so thick heavy black paint is the rule and if you ask to leave it off the answer is no citing that they are all done in batch's . At least that is the scoop based on numerous posts on the concours forum . I haven't bought any from Eaton for years as I have found the VC Mustang to be be comparable in appearance ,quality plus I would rather give the business to a forum member all things being equal or better.
Thanks Bob. I agree on giving Virginia Mustang our business. When I bought my springs they were the only ones I was aware of that did them for a specific application and not one size fits all. NPD is local. So was Eaton so easy choice at the time. I bought the clamps from Shelby Parts a few years ago and plan on restoring mine as the ride height was still good with my old springs. Retirement project: Replace/restore as many original parts on my vert to bring it up to SAAC gold standards sans the gold stripes and underneath paint wise. My rear frame rails and floors look as good as the top side still.
Quote from: shelbymann1970 on June 14, 2022, 10:56:42 AM
Thanks Bob. I agree on giving Virginia Mustang our business. When I bought my springs they were the only ones I was aware of that did them for a specific application and not one size fits all. NPD is local. So was Eaton so easy choice at the time. I bought the clamps from Shelby Parts a few years ago and plan on restoring mine as the ride height was still good with my old springs. Retirement project: Replace/restore as many original parts on my vert to bring it up to SAAC gold standards sans the gold stripes and underneath paint wise. My rear frame rails and floors look as good as the top side still.
Gary ,I hope that you kept your original leaf's besides the bottom plate. All of the leaf's are a different length on the S9MS set compared to a regular mustang. the only one in common is the main leaf with the bushings. Consequently a replacement set is not worth restoring unless you can do it for less plus show detailing/bottom plate then what a good replacement set would be.
Quote from: 69 GT350 Vert on June 13, 2022, 06:59:34 PM
Quote from: Bob Gaines on June 12, 2022, 04:08:22 PM
Quote from: 69 GT350 Vert on June 12, 2022, 03:52:16 PM
I found this information in a General Spring application chart. It shows the aftermarket part number of 42-503 as the correct spring for a 69 Shelby.
I don't know if this information is accurate or not.
That would be a replacement substitute no better then the universal replacement that Ford sold for decades to fit just about all Mustangs 65-70. I have bought ones from Virginia Classic Mustang that look the same but are a little less then Eaton plus Brant is a forum contributor. Basically you get a generic replacement regardless of who you buy from. Eaton covers their springs in a thick black paint which for a driver is fine because they will be well protected from the elements but get ready for a major amount of work trying to get all that paint off if trying to make them look close to original. Virginia's still have paint but not near as thick. Regardless of where you get a replacement the leaf's will not all be the proper length and the bottom leaf will look different . At least compared to a assemblyline leaf . ;)
Based on info in an older thread, the VA Mustang leaf spring is industry replacement number 42-437, which has free arch of 6.25 inches. The 42-503 spring listed by General Spring for the 69 Shelby has free arch of 3.75 inches, which would provide a lower ride height. It is likely the spring rates are also different. Fortunately, my early original leaf springs stamped 5RS68 are not sagging. However, the original front and rear bushings are cracked. I should pull them off and change them, but the car rides nice as is so I haven't messed with them.
Interesting specs from the books. Based on the free arch measuring technique discussed on Eatons YouTube presentation https://www.youtube.com/watch?v=Gz1FE4QaqZU those posted book figures do not jive with the real world. I laid out 6 separate will used original S9MS leaf springs that I have extra and got various measurement varying from 6 1/4 to 6 9/16 . FYI I know that they are original because besides the flat bottom plate the other two leaf's are a different length compared to a regular Mustang set. I assume the variation between all of them was due to age fatigue. I also measured a pair of well used B2 leaf's that I had extra for comparison and was surprised at their 5 7/16 free height.
Quote from: Bob Gaines on June 14, 2022, 11:27:43 AM
Quote from: shelbymann1970 on June 14, 2022, 10:56:42 AM
Thanks Bob. I agree on giving Virginia Mustang our business. When I bought my springs they were the only ones I was aware of that did them for a specific application and not one size fits all. NPD is local. So was Eaton so easy choice at the time. I bought the clamps from Shelby Parts a few years ago and plan on restoring mine as the ride height was still good with my old springs. Retirement project: Replace/restore as many original parts on my vert to bring it up to SAAC gold standards sans the gold stripes and underneath paint wise. My rear frame rails and floors look as good as the top side still.
Gary ,I hope that you kept your original leaf's besides the bottom plate. All of the leaf's are a different length on the S9MS set compared to a regular mustang. the only one in common is the main leaf with the bushings. Consequently a replacement set is not worth restoring unless you can do it for less plus show detailing/bottom plate then what a good replacement set would be.
I learned before I restored my car to keep all my old parts. I posted a pic of my leafs. I still have my original doors also. Original trunk lace. "original Konis...." My car was sold new on Sept 30th 1970 as per the title on the car and my Konis are date 6/70 but of course could never prove they were installed when the car was new since Claude "lost' all his paperwork on those cars(his memory failed him on what he did with them he once told me).
second pic is when new 3rd pic is 1989. The stance really hadn't changed.
Is there any historical data on who actually manufactured the springs that A O Smith installed? Were they shipped by a Ford supplier or did A O Smith contract them?
My experience with Eaton Detroit Spring was similar to Bob's - "knew everything."
QuoteWere they shipped by a Ford supplier or did A O Smith contract them?
I believe some were supplied by Ford and others were actually installed by Ford.
In regards to Eaton, like a lot of other vendors, everyone's experience can be different. I am also a member of the Fairlane Club of America and when the club president approached them years ago about getting the correct springs built for late 63 and up cars, they were very cooperative. There was only I believe a 1/2 or 3/4" difference in were the locating pin was located (had something to do with (NHRA super stock wheelbase rules). Bob supplied a sample and they matched them exactly. Like I say, every experience is different.
So circling back around to the OP's question, can anyone provide some specs on the differences between the original Shelby springs and the replacement 42-437 Mustang springs i.e. difference in length of 2nd and 3rd leaf that was mentioned? Is there a difference in the length from the pin forward or the pin rearward on those 2 leafs compared to the Mustang spring? Length of the bottom flan leaf? Bob, your measurements of of 6 1/4 to 6 9/16 would put them in the ballpark of the 6 1/4 spec'd from Virginia and the 6 5/8 spec'd from NPD/Eaton on the 42-437 Improved Handling spring . Most of the vendors with the Improved Handling 42-437 springs quote a rate of 114 lbs. One vendor has quoted the rate on the 42-503 at 152 lbs. I called General about the 42-503 to see what the spring rate was on that spring but person was having computer issue at the time so I didn't get a number from them. I am calling them back later today to get an update. If the 152lb number is correct, that would definetley be a stiffer spring which could have something to do with the lower free arch number.
Quote from: Dennis Y on January 05, 2023, 01:27:52 PM
So circling back around to the OP's question, can anyone provide some specs on the differences between the original Shelby springs and the replacement 42-437 Mustang springs i.e. difference in length of 2nd and 3rd leaf that was mentioned? Is there a difference in the length from the pin forward or the pin rearward on those 2 leafs compared to the Mustang spring? Length of the bottom flan leaf? Bob, your measurements of of 6 1/4 to 6 9/16 would put them in the ballpark of the 6 1/4 spec'd from Virginia and the 6 5/8 spec'd from NPD/Eaton on the 42-437 Improved Handling spring . Most of the vendors with the Improved Handling 42-437 springs quote a rate of 114 lbs. One vendor has quoted the rate on the 42-503 at 152 lbs. I called General about the 42-503 to see what the spring rate was on that spring but person was having computer issue at the time so I didn't get a number from them. I am calling them back later today to get an update. If the 152lb number is correct, that would definetley be a stiffer spring which could have something to do with the lower free arch number.
Mine are buried in my attic. When I get a chance i'll pull them out but mine are for a GT350. I will not assume the same leaf lengths as a GT500. Bob?
Quote from: shelbymann1970 on January 05, 2023, 01:42:31 PM
Quote from: Dennis Y on January 05, 2023, 01:27:52 PM
So circling back around to the OP's question, can anyone provide some specs on the differences between the original Shelby springs and the replacement 42-437 Mustang springs i.e. difference in length of 2nd and 3rd leaf that was mentioned? Is there a difference in the length from the pin forward or the pin rearward on those 2 leafs compared to the Mustang spring? Length of the bottom flan leaf? Bob, your measurements of of 6 1/4 to 6 9/16 would put them in the ballpark of the 6 1/4 spec'd from Virginia and the 6 5/8 spec'd from NPD/Eaton on the 42-437 Improved Handling spring . Most of the vendors with the Improved Handling 42-437 springs quote a rate of 114 lbs. One vendor has quoted the rate on the 42-503 at 152 lbs. I called General about the 42-503 to see what the spring rate was on that spring but person was having computer issue at the time so I didn't get a number from them. I am calling them back later today to get an update. If the 152lb number is correct, that would definetley be a stiffer spring which could have something to do with the lower free arch number.
Mine are buried in my attic. When I get a chance i'll pull them out but mine are for a GT350. I will not assume the same leaf lengths as a GT500. Bob?
Same same Gary.
General also quotes a spring rate of 152 for the 42-503.
the 3 "Shelby" leafs are 20", 33.5" and 44.5" approx as I'm up in an attic by myself trying to measure end to end with a tape measure on an arc.
Found this write-up from the Boss Registry. Goes back a ways but provides some interesting information. The S9MS 5560A that is on Gary's spring matches what is shown on the chart for the Shelbys. Interesting that the spring rate is lower on the Shelby yet the load rate is higher than that of the Boss. Shame that free arch is not noted but as was mentioned, they used different parameters back then. If you use the same formula as shown for the Boss spring, the Shelby's 750/135 would mean a free arch of approximately 5.5 inches which is still short of the numbers Bob came up with but definitely better than the 3.62 for the Boss. I am going to try and enlarge the engineering drawing at work on Monday and still have it somewhat readable. We'll see what happens.
https://www.boss302.com/spring.php