My potential compression ratio

FWIW, I just noticed you are using the Comp XE268H cam. My 360 has KB107 pistons at zero deck, 64 cc magnum heads and the FelPro 1008 head gasket with the XE268H cam. I plugged in the numbers into the KB calculator and came up with a dynamic compression ratio of 9.16:1. The actual cranking pressure measured with a compression tester is 185-195 psi.

I run 33 degrees of total timing (magnum heads require less than open chamber LA heads) with it all in at 2500 rpm.

Anway, I run it on 89 Octane and have in a pinch run it on 87. I have to be careful putting a load on the engine below 3000 rpm or I can just detect a little detonation with 87.

The magnum heads with a proper quench and the high swirl ports makes for a very detonation resistant engine. I would think you will be fine with 91.