Many guys post here their car is running fine on pump gas my RAIV had a moderate amount of overlap also. I'd bet a dyno test with race gas they would see a need for more octane.
Dish options-custom from Ken etc, in whatever size you want. KB hypereutectic-be sure to gap the top ring EXACTLYto thier specs or break ring lands., Get a F-M(TRW) forged 2262 machine a 2.8x0.100 dish on the lathe -the old HO dished Ken Crocie used to sell., I think that figures around a 9.9cc dish. Bet by the time you add the machine work Ken can get you a better deal.
Other option is opening the 670s chamber closer to a later 68 and up chambers. Tom Wilhite has a special Bridgeport cutter for this. That should get you the 10cc easy if you don't need a fresh rebuild. I always thought opening them to copy a late model Fast Burn SBC chamber(biased toward the exhaust) and keeping some of the quench would be a good thing to try on 670s and drop the compression.
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