How Much Can Heads Improve Compression???

If I have my notes right, you're going to drop about 7-9 cc's going from the option A 675 to the option B 920 heads. (The former have 65-66 cc's typical and the latter in the upper 50's I do believe; anyone else. please chime up if I am wrong. Assumes nothing has been shaved, which may not be true....I would just CC the 2 heads to be sure.)

Are these pure flat top 318 pistons with no valve reliefs, or 2 or 4 valve reliefs? Any pix of the piston tops? Can you provide the piston top below deck measurement?

Guessing on pure flat tops with no valve reliefs, and a 1.74 compression height which puts the piston about .080" in the hole, the computed CR changes from 8.3:1 to 9.1 by changing the head chamber volume from 66 to 57 cc (head A to B), with a compressed gasket thickness of .032.

Valve reliefs will drop those numbers so you need to help us on that....I'd CC those too. (Tilt the engine, use light oil in the burrette, Mazola, whatever...)And a piston deck height.

The heads look pretty nice to me.