340 Piston and Valve Question

OP, set it sounds like you have open chambered heads. The pistons ought to stick up into the open area all around and have no clearance issues to the head. You can just put a piston at TDC, sticking out, and set the head on the block with no gasket; the head ought to set flush without hitting the piston. You can put a thin piece of clayon the piston head when yo do that and see what thickness you end up with.

Sounds like your heads may have been milled something like .030" based on your cc numbers. But even then the recess in the head will be something like .080" deep, and that, plus the head gasket thickness, will keep the pistons far away from the head.

Just curious: What .030" pistons do you have?