Guess The Compression Ratio

FWIW, my 360 with magnum heads is 10.6:1 and is not octane sensitive and will run just fine on 89. BUT, the closed chamber magnum heads make of a detonation resistant engine.

Also what is the total advance you are getting from the engine? If the distributor has not been recurved and you are running additional intial timing then you likely have too much advance.