Can't say for sure about the 64 rear on a 65. Although a 66 will physically bolt up (you might have to use the 66 bumper braces, the turn signal openings are different and you have to use 65 with a 65 and 66 with a 66.
A 66 rear bumper on a 65 will require slotting a couple of the bumper bracket holes, as they offset them from the factory so the 65 won't bolt up to a 66, just like they did with the tail light housings.