Stock Rear
Never knew the abody had the 8-3/4 option for 65 and 66. How come the 273 4bbl did not come with them?
Because the 273 4bbl was only rated at 235 hp and didn't need one... So they were standard 7 1/4" but optional 8 3/4"...
Once you get in the upper hp, you then need to upgrade the 7 1/4" to 8 3/4"... I build a 318 for one of my 68 Barracuda fastbacks with only a 340 cam, 9.2 compression, 4 bbl, dual exhaust with stock exhaust manifolds and broke three 7 1/4" axles within 4 years on a daily driver - not beat... I replaced it with 8 3/4 and drove that car to 500,000 miles with no more axle problems.... The 7 1/4" axle can't handle a 318 with 340 parts it's so weak, but it's ok for a stock slant 6 or 273...
I have a 67 Barracuda fastback 273 4 bbl Formula S that came well loaded with options, being factory trailer tow with 8 3/4" 3.23 sure grip which was part of the trailer tow package...
My dad had a 67 Barracuda Fastback 383 4 speed back in the day, I was too young to know anything about them back then, but I would think, that it was a factory 8 3/4" sure grip axle... He got run off the road with only 6500 miles and the car was totaled back then...