I would say that it depends on what pistons are in the 428. If it has factory 13cc dished pistons, it should be right at 10:1. I have actually had good luck with my 428's at near 10:1. I run 670 heads on one and it never pings on 91 octane. Another engine has 614 heads and runs good on 91 octane. I ran another one for years with 62 heads and when I had it back out of the car, the bearings all looked brand new and there were no signs of detonation. It ran on 91 octane as well. You would just need to be careful as to what camshaft you choose for the engine so it won't build a ton of cylinder pressure.
If you run the 6X4's and your engine has the factory style dished pistons, compression would be in the low-mid 8's. Obviously it would run good on pump gas but maybe not the best for performance.
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