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Bob --- sorry, maybe I mis-phrased my question as I did not know if anyone had the measurements for the Boss 302 springs and was just musing which springs were made first.

Page 25 of this Spring Research version has the 1970 B302 measurements. https://springresearch.org/pdf/volume_18.pdf

These were written specifically for aftermarket manufactures to build to OEM (or very close) specs. Note that the B302 spring is assigned the same 42-503 number as the GT 350/500 replacement however the free arch of the the OEM is 1/4" different.

I have no doubt the OEM springs are visually different as pointed out already.

69 shelbys were designed to have the new wide low profile f-60-15 polyglas gt tires and early shelbys had tractions bars so i think the new leafs were designed for better traction with new wide tires. 69 shelby leafs only used 2 round hole (not square hole) spring wraps not 3 like other year springs and bottom leaf lays flat and dont use a sring wrap. The front eyelet had white paint sprayed on the front end and the later cars started using c9zx bottom leafs along with the s9ms and i never could figure out why #s change as gt 350 vs gt500 vs 4-speed vs auto as nobody that i know of has figured out a pattern but the very early cars had 5rs68

Not sure how much this helps but page 35 calls out a 69 GT350/500 spring with less free arch springresearch.org/pdf/volume_17.pdf

That's who I used too

Found the place Brant and VA Mustang used to source correct looking springs from. Worth a call to see who they supply to now https://www.johnrspring.com/leafsprings.html

4 leaf, see page 32 of 77 https://springresearch.org/pdf/volume_17.pdf

Virginia Mustang used to sell springs that looked like the originals with the rounded ends. I forget the name of the place they were sourced from but maybe Brant will see this and remember, it wasn't Eaton. I spoke with the manufacturer and they were built to 42-437 spec which was the most generic. I have a friend who owns a spring shop and de-arched them for me.

Plus a Shelby on a trailer

The few I have of 69/70 cars

I looked at an early unmolested 65 K about 25years ago and I recall the attachment point on the frame being more forward than what is normally seen on 65 and 66 cars. If you don't see it in the normal place, it may be further up the frame toward the axle.

At the time I didn't know about the Arvinode system so I didn't pay much attention to rest of the exhaust.

These guys make a concours reproduction system and the rear seat hangers look to be different and maybe mounted more inboard. https://waldronexhaust.com/product/1965-69-ford-mustang-concours-model-arvinode-dual-exhaust/

There is also this MotorTrend article with a picture Charles Turner provided from a system he removed. Try reaching out to see if he can shed any more light on what to look for https://www.motortrend.com/how-to/mump-0907-arvinode-dual-exhaust-1965-hi-po-289-engine

Good luck

but the first most glaring issue is that in the O.P.'s original posting the .522" dimension as this isn't so typical

         

100% this ^


FWIW, the Crane teflon seals I installed in 1986 are still functioning flawlessly today. That includes every type of driving from SAAC open track days to just driving around around town.

Boss has definitely been machined. I used Crane 99820 but that was almost 40 years ago. Not available at Summit but has the specs you're looking for https://www.summitracing.com/parts/crn-99820-16

Some on EBay https://www.ebay.com/itm/306185233774

Never heard of this claim before outside of R models. Have I been living in an alternate universe?

https://bringatrailer.com/listing/1966-ford-mustang-827/?utm_source=Iterable&utm_medium=email&utm_campaign=campaign_2648441


"Going from memory here on the GT 350...full bench breakdown, standard bores honed, stock pistons retained but with different ring gap, more precise head efforts on the valves, springs, and seats. Crank polished and balanced and different cam grind. Probably more but I would have to study data."

Hmmm  https://hipo-mustang.com/thread/7131-6r09k130238-engine-block-found/?postID=56038#post56038

I replaced one about 5 years ago. If I remember correctly, it was the Chrysler pierce nut https://www.amkproducts.com/bulk-fasteners-listing/?codeforlookup=7135

Just wait until some day when you're cruising around 50 mph and the locker locks. You'll think you snapped an axle but you didn't.

Another casting difference is the distance between the red lines. HiPo carbs have less material to prevent interference with choke hardware.  Some fakes cut it back and some don't. Either would still have witness marks from being filled.