What is your static compression ratio? If you have detonation due to too high a compression ratio for fuel octane and air temp you'll get a whole bunch of crankcase pressure from that too. Or that is the 'experts' evaluation/guess to my former motor bawling Hard/expensive lesson learned. I have a Milodon 31151 pan as well but does the 31580 have a divider/seal plate between the bottom of the engine crankcase and pan top? I wonder about the pistons pushing air against that pan plate? Have a brand new vacuum pump assy waiting on new 493" motor (with lower compression) and new header combo.