273's came out in 64. It could be. Look at the side of the block . The engine size will be cast into the side of the block. Looks like something later model in there 1967+ valve covers and the alternator is on the wrong side.
True, I heard somewhere that it’s on the opposite side because it’s classified as a race engine though. Again I can’t confirm that in any way because I haven’t found much about it yet online :/