Are Blue Felpro Head Gaskets the Same for 273/318/340?

While they can be interchanged, not all "blue" Fel-Pro gaskets are the same. The 1007 is .039" thick and has a "fire ring" built in. It's their best head gasket, especially for higher performance & higher compression applications. They have other part numbers that vary in thickness & probably bore sizes. It depends how particular you are about the compression you're shooting for.

A bigger gasket bore than needed contributes (very slightly) to less compression & efficiency also.