302 casting on a set of 318 heads

FWIW.... I stopped worrying over that factor quite a while back. For a .039" thick head gasket, going from the standard Felpro 4.18" diameter down to 4.00" (you can't go to the exact bore due to misregistrations of gaskets and holes) raises SCR by 0.07 points. The numbers are similarly tiny with .028" or .050" head gaskets thicknesses.