Drive Shaft on a '65 Fastback

Hey guys! We were getting ready to start dropping the engine in and were looking at all the related parts when we noticed our drive shaft had a bend in it. So we looked around and found a '68 drive shaft which we were told could be interchangeable. However, the base of the shaft is different, and the diameter in general is larger than the 65. It is the same length though. So I guess the basic question is, can I replace the '65 drive shaft with the '68? Thanks!