What Differential Do You Have in Your 1965 Barracuda?
This discussion is something there is no definitive answer for. If your 65 was built with a HP273 you "probably" got a 7 1/4. I haven't seen anything that stated that after X date or X VIN the cars got a 8 3/4. If your 65 has one, there is a chance it was replaced at the dealer. THere could be a slim chance (very slim) that it left the factory with one possibly very late in the production year. The Build sheet "should" tell. THe fender tag will not.
Speaking of which, the rear axle sales codes are from 371-379 and all that is listed is ratios and not rear axle models. That includes all cars. Sure grip is a 592 option.