My cooling system is a no pressure system. Meaning that even if I put a 7 lb. cap it would still overflow out of the vent tube on top of the top tank.
The pressure rise increases the boiling point of the water, allow more heat transfer without boiling over. There is a 4 lb cap they intorduced in 52, with no other changes to the system.
I would check the water pump. First run the engine with the radiator cap off. When the engine is warm, does the water barely move? If so, check the pump impeller. Another problem can be the water moving too fast. I'm thinking Patrick's recommends a large pulley, but I'm not sure of the size. A smaller pulley will make the pump spin fast, moving the water too fast for good heat transfer.
Does water flow out the block drain? At the rear, drivers side, a few inches above the bottom of the block. If not, your water passages are pulgged, and need to be cleaned out.
Also, before pulling the head, try torquing the head bolts again. See if any are loose. That might be your problem. If not, then I would pull the head.
