This may be a cheapo fix, but my 66 had the exact same challenge when I inadvertently added 87 octane. It was a bone stock 72 Pontiac 350.
Someone suggested adding an octane booster the next time I filled up with 93. The particular brand was NOS Octane booster. I think I was buying the of road version. I certainty helped. The motor was quiet again. The catch was I had to use it with each fill up at $9/bottle.
A couple of years later we switched out the intake and carb and the need to the booster was gone. That says something, not sure what though.
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