The early Pontiac V-8's (pre 1965) can be more work replacing the engine with a later model. The starter was attached to the Transmission until at least 1963 (I have never owned a 64 Full size car so have no 1st hand experience with them, but my 63 Bonne had it trans mounted, and my 65 had it block mounted) So starter mounting can be an issue, also if you get a later engine make sure you get all the accessory mounting brackets with it as they change drasticly over the years, and some Block/Heads do not have the needed mounting holes for the brackets.
And avoid 67-69 Firebird V-8's as they do not have the needed motor mount holes drilled and tapped in the block, any full size 1965-75 or 65-75 Midsize or 1970-75 Firebird engines would be your best bet. But all may require the transmission to be swapped as well. And this may not be a bad thing as the TH-350 or 400 are stronger, lighter and cheaper to rebuild than the stock 4-speed Turbo Hydromatic or Slim Jim trans you car came with.
