DGC, piston height is year dependent. Later years are low comp. engines while earlier years are positive deck - high comp pistons/ratio's.
If he goes to a surplier of pistons and doesn't make sure of the piston, he could get a low ratio piston, way in the hole.
71valiant, what bobscuda67 said and what DGC said.
If you go with an aftermraket flat top piston at zero deck and a typical 0.040" thick head gasket you will have about 10.6:1 compression.
This is a good way to go about it.
If you have a positive piston in your 340 now and intend to keep it, you may have to find a much thicker gasket to allow the closed chambered Magnum head. Look to Cometic gaskets for a custom thickness and proper bore size.
Remember, no closer than .040 for saftey and long life.