273 302 318 340 build

OK, on the open chamber heads. In that case, the head gasket thickness is not critical for the piston-to-head clearance. So the MR gaskets would work and be pretty close to the stock thickness. I would not worry overly much on the head gasket bore being about .070" larger than the cylinder bore; you are not going to be pushing this to the limit the CR loss will be teenie-tiny and there should be no realistic issues with that small crevice.