64-72 A body rears interchange as far as attachment points. There might be minor differences in width, but they will bolt in. The Chevelle rear will probably be C clip. The Buick rear will be bolt in axle. What's wrong with the 65 Buick rear?
Wow no buick "purist" has commented that you might as well rebadge your car as a chevelle. You got lucky op this place is tough when it comes to asking about swapping in Chevy parts aka premium parts.
Apples and Oranges, swapping in the heart of another beast changes the identity of the car. It is likely no one would notice a different rear unless you brought their attention to it. Buick rears are mostly bolt in axle which is a much better method of axle retention than C clip. Lots of guys went to the Chevy 12 bolt years ago, and Canadian Buick Skylark came with them from the factory.
