Even though your post was directed to someone else, I thought I might chime in. I have spent alot of time with a set of RPM heads(70cc advertised) cc'ing, and trying to equalize the chambers. Unfortunately I didn't measure them before I started blending sharp edges around the seat inserts, and polishing to give you an idea how much of a difference it made. I know the polishing is just going to get covered with carbon, but from what I've read it will still help a little to reflect heat back into the chamber and avoid hot spots.
The point I was trying to share is that it takes a lot of grinding to make much of a difference. To gain 5cc equally on all chambers without compromising their integrity would take an extremly long time if even possible IMHO. You would have to remove alot of metal that might weaken them or hurt their combustion efficency, but JMO.
I believe you would be much better off buying another set of pistons with a 12cc "D" dish, but JMO.
I'm in process of building a 383 much like the one you described. 0.040 overbore, 5cc vr pistons, 4.166" x .039" gasket, and zero piston to deck height with 71.4-72.0 cc chambers. I get around 10.2-10.3:1 CR. I trying the groove thing for all the insurance I can get, even though I have no proof they work. Forgive the spelling.