65 Bcuda (please educate me)

You shouldn't have to replace your differential, who said that?

If you're using your original early A-body driveshaft you'd have a problem with it mating to a later style tranny because the early driveshaft uses a ball and trunion style u-joint/coupler. You would want to switch over to a Spicer style (see this article http://www.earlycuda.org/tech/ujoint.htm)

Otherwise what's the problem?