I have personally run 11.6:1 in an iron block alum headed hemi on the street for years. My current all alum 605 is 11.2:1 and neither are engine had any issue with pump gas. I know of multiple folks running 12:1 in big Hemis on pump gas.

So if your starting from scratch, go big or go home. Use the thinnest .027 Cometic gaskets at 12:1, and later, if it does give you an issue, switch them out for thicker .041's and back it off..

Now with all this dizzying thoughts of massive Hp improvements, remember, you need good eye on your timing and your carb jetting.

If your going to put some 25 year old reproduction carbs on it and a stuffy dual plane intake....You may want to reconsider.

PS...My 800hp 605 Hemi is for sale