A advise

Depends on what you're trying to achieve. If you want more horsepower a 318 is probably the best bang for the buck. It's also easier and cheaper to get pistons for. If you find a cracked head on your '64 (assuming it's stock and has the 315 casting heads) finding another one is getting a bit hard to find these days. Going with the later heads is an option, but, it would require a later intake to match. Everything else internally is pretty much the same between a 273 and 318 interchangewise. If you're looking for max MPG, stick with the 273.