I've heard that too, but, I've still never seen proof of a '65 model with a factory installed 8 3/4. Dealer installed, yes, lot's of those due to breaking the 7 1/4. I'm not sure what would be needed as proof however. An original order sheet would probably have that. The build sheet MIGHT indicate one.....so MIGHT the window sticker. 409 code is a special order rear. The axle ratio could be a clue also as some ratio's weren't available in some rear ends. The driveshaft code on the build sheet would probably indicate which rear it came with too as the length would be different. As for it being standard on a '66 4 speed car, no. I've personally seen two original '66 FS cars with a 4 speed and a 7 1/4 SG rear. All my manuals only list a 7 1/4 rear axle housing and axle shaft for a '65 model. My early '66 Barracuda ( 8/17/65 build date) came with what I believe is a factory 8 3/4. It didn't have a window sticker or build sheet when I bought the car in 1980, but, it doesn't look like it was ever changed and the ratio code matches the IBM card from Chrysler Historical.
