Blowing out block warmer plug?

I have a 95 12V auto that is almost entirely stock except for 18.5 timing and blocked off wastegate. Today it blew out the block warmer for the second time since rebuild about six months ago. I barely drive this thing and as long as the gauge is working correctly, I'm only hitting about 15 pounds of boost. SO, what could be causing this to happen?

How is the radiator cap?

This engine is in my 97 F350 that had a 460 before, the radiator was fine as far as I know. Are you saying it could need a different cap with a lower setting?