273, or 318?

This entire proposition is a little vague. First, are we talking about a 273-4bbl (somewhat rare, high compression, special cam), or a 273-2bbl (common, anemic)? If you are thinking of building a performance motor out of a 273-2, you would be better off starting with a 318 — no substitute for cubic inches, everything else is similar, and the 273-2 adds no value. But if you have an original 273-4, you will have to put some work into a 318 just to match it, and the question of originality enters into the equation. Right now we are unclear about your starting point.