When you pull the heads you will see that your pistons (1.720CH)are almost exactly 0.100" down in the block. This is about 20cc. The 1121G head gasket is maybe 7 cc. The heads are 72cc. So you're looking at 99cc above the piston.

The swept volume of a stock bore 318 is 652cc. (652 + 99) / 99 = 7.59:1 compression.

IMHO you will be building a dog. You'd be much better off with a set of stock Mag heads.

What that engine needs is a 3.51" crank to go with those J heads. That'd get you some compression and a few extra cubic inches to play with.

R.