I have a 400 30over, I think a 551 mopar purple shaft solid cam,
383 crank, aluminum heads and intake, holley 750,
What should I use for a rad cap pressure wise,
Thanks for any suggestions.
Dave

A regular 16# stant will work fine. You want to keep it from boiling cuz when the coolant turns to steam the psi goes sky high & the boiling point goes up ~3 deg for each psi of pressure in there (like a pressure cooker on your kitchen stove, you twist/open the lid & you see your beans start to boil). but 212F (at sea level) + 48 degrees, you shouldn't even come close to that.

You can use a lower psi but be sure to have enough recovery tank like a factory 1.You can start with a lower psi like a 12# if you think for any reason things are questionable in the system.