cracked 273 heads

The head gasket leaves a lot of dead space, but if the gasket is sealing there should be no problem. Do you have a picture of the gasket and the block's gasket surface where it blew?
The issue is the other 318s I think with the exception of some '68s have open chambers which will drop output further. Not tremedously, but drop it, plus they don't have the induction hardened exh seats. So I'd go 302s and get an intake for it. As said the manifolds will clear - after a little clearancing. I gotta beleive there's a 64 or 65 273 2bbl somewhere closer to you too. They're free or cheap usually for the whole engine.