You can't run flat tops, 4.21 stroke with 48s on pump gas. Never going to work.
I would not push it past 9.5-1 static on 91 octane.
Even back in 1970 with PMD 455s with 64 heads, 10-1 CR the dealerships were getting come backs from customers breaking pistons not running premium fuel. Not sure what the octane rating on premium was back in 1970 but I bet it was higher than 91-93.
Whatever you do, get a custom pump gas cam to give your engine a chance of living.
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