I was driving my '55 Bird yesterday when the engine started puking antifreeze.  Pulled off the side of the road an found a freeze plug laying on the passenger side of the frame.   Two weeks ago I upgraded this car to an aluminum radiator and electric fan.  The car runs cool all the time and it's was a huge improvement.  I'm sure the new radiator cap has a much high pressure setting.  Could that cause my freeze plug to blow out?  The engine was rebuilt by a previous owner.  The block and freeze plugs are not rusty.