Given the options you mentioned, replacing the 270 with a 302 is a no-brainer- - - -it's virtually a bolt-in swap. If you happen to find a military surplus 302, you'll need an electric fuel pump or the hole for the mechanical fuel pump will need to be cut out at the proper location in the block, and there's a place on the block that needs to be ground away to clear the starter, but both of those are relatively minor modifications compared to adapting a different block bolt pattern. Good luck!
Jerry
