We know the color is wrong, but the seller states the transmission is an RUG-S1 but it seems to look like it has a full-size tail-shaft on it.
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There is a shorter pickup truck tail shaft but so far as I know Mustangs / Cougars and full size Ford products / Mercurys used the same length tail shaft and case.
A true RUG-S1 transmission would fit a '67 GT500. This is what was installed in my '67 GT500, 67400F7A03040.
Yes, it does seem to present as a "long-shaft" example vs. the "short-shaft" as should be in the Mustang chassis.
Also, I believe the shifter should be mounted in the rear-most provision on the ("short") tailshaft vs what appears here where the shifter in mounted forward on the ("long") shaft. :)
Scott.