magnum heads do offer better flow put the close chamber heads could be a problem with a 340 if the 340 has positive deck height pistons...you will need thicker head gaskets which will negate the quench...
Does not compute... The quench gap will be there whether the piston tops are at zero deck or above deck. Just set it with the head gasket thickness. We have piston .005" out of the hole and a .045" thick gasket for a quench gap of .040".