OK, there are plenty of suggestions for the difference in deck height. I have nothing to add there. But to address your second question: Yes you can. Factory 340 high compression pistons (68-71) came out of the hole .015-.025 and they cleared the open chamber X and J heads even with thin steel shim head gaskets. Those pistons are the reason why Edelbrock offers their 340 aluminum heads in a large chamber version which is just the regular RPM head machined the diameter of the cylinder some .060 inch - just to clear pistons with a positive deck height.