I built a 496 BBC blower motor, gen iv merlin block and edelbrock rpm 60559 heads and at first start up, only ran 5 minutes and I blew out the heater core. I cant find the problem, I took off the intake and gaskets look real good. compression test was all good at 150psi. oil looks milky grey white. my bore is 4.310 and I used a felpro 1027 headgasket, I was told I should have used a 10171 and my problem would not have happened.