Pressure test the cooling system with the spark plugs removed.
What head gasket are you using?
Might be a cracked head or cylinder wall...
Head gaskets are the weakest link on SB Mopars, especially while running boost.
How much PSI of boost?
I am not sure about the head gaskets. My dad helped me build the engine and he passed away a few years ago. I do recall we got them specifically for high performance / boosted applications though. They are not stock gaskets.
It would be about 6-8lbs of boost, pretty mild. I never really got into it enough for it to pull into boost too much to be honest.
