Edelbrock RPM Head compatability issues.

Here is a compression ratio calculator:

http://kb-silvolite.com/calc.php?action=comp

What you have bought is the closed chamber version of the edelbrock heads which is definitely better then the 340 version because you can build an engine with a quench combustion chamber. A quench combustion chamber will greatly resist detonation allowing you to run on pump gas with good compression. What you want to do is have pistons that are zero deck and then run a .039" inch head gasket for good quench.

In some cases the positive deck pistons can be milled but I'd call the piston manufacturer or a reputable machine shop to determine if that is possible with what you have.

If you can't mill the pistons and they are above the deck like a stock 340 pistons then thicker head gaskets will give you the piston to head clearance to run those heads.