Well got old motor out,man was the hypermax turbo a pain to get off....new motor installed but not hooked up still have to order some parts from hypermax.... Ever see a motor crack one head between every valve?
Sounds like alot of pressure from the turbo causing the heads to flex. It could also be excessive heat from a lean condition. (common with a forced induction engine not properly tuned) My suggestion is thicker castings/aftermarket heads and then stud everything with ARP studs. ...and then make sure you have enough fuel getting to engine.