I have the same rough combo in my '65 GTO on the 389 I built in 1981. It has about 50,000 miles on it. Forged flat top pistons, milled heads (about 11:1) cranking compression 175 all cylinders, #77 heads, Sig Erson cam with a lot of overlap and lift. I need to run race gas to keep out of detonation. Sounds like me, you simply have too much compression. You can: run race gas, disassemble and install dished pistons, or, install bigger chambered cylinder heads or 72 cc aluminum heads. Lots of people seem to think a bigger cam will get you out of detonation, but it will not. It will only change the rpm and load it detonates at.
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Jeff 
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