As an update to this, just for future reference. I modeled up the combustion chambers in SolidWorks and played with different material removals. Pretty much proved out what was suggested that I'd be looking at something in the .030-.040" range to get where I wanted. I split the difference and asked the machine shop to knock off .035".

Combustion chambers now measure right around 51.5 CC's. I forgot to measure the overall thickness now, but I'd GUESS that it's been shaved about .040" total (first pass to clean it up + second pass to drop the volumes.

Feels good to have NEARLY all the machine work done. Block is honed, decked and tanked, valve seats are replaced, valve guides are in, crank journals are polished, head porting is done, new pistons and wrist pins are fitted.

Last hanging chads are getting the flywheel surfaced, and getting the cast iron manifold spot faced to use with the aluminum intake manifold. Having a tough time finding someone with a long enough end mill to get down between the runners.