Small block heads

I say dont mill. If you really want to use the 340 heads get taller pistons and a thinner head gasket. Take about .010" off the heads to get the chambers down to 69cc, a piston that sits about .011" in the hole and mopar gaskets that compress .024"-.026" with a 4.060" bore and you'll get about 9.5:1 compression. that's if you really, really want to use the 340 heads.