My 360/300HP magnum crate was a measured 8.8:1 when I pulled the heads for porting. I milled .010 and went from the OEM .040 gasket to .028. I'm now at 9.3:1, run 27* intial 34 total on 89 Octane at sea level.
Next time you need gas put a 1/4 tank of regular in and drive it. Accelerate in 3rd gear and see if it pings. You can also pull the plugs after driving and check the porcelain with a magnifying glass for tiny silver specks.
My guess is if you are not running a carb with a super lean fuel curve the regular will work.
The horror stories of cylinders pinging in Magnum motors is mostly due to the beer barrel gasket sending oil in the cylinder. Also a dirty fuel injector will make a cylinder lean and the ECM only gets an average from the O2.
