the 4# rating should be fine w/ these old rads [even rebuilt] and if you're not blowing a lot of coolant out the overflow when it's 'hot', things are fine - the 7# cap will allow higher pressure before blowing off, which might be useful when it's runnin a bit warm, but running the 7# you're more likely to spring leaks on an old rad - nothing to do w/ boiling, just that the coolant changing temps from rad to block results in rapid temp change expansion
in my experience, the 235's are cold runners, and using a 160 [which allows cooling to start at that lower temp] normally results in a 235 running cold - 180 is what it should have, to keep the temp above that - I agree ya got something wrong someplace if you're having problems w/ a 4# cap and 180 thermostat
are you sure the water pump is pumping well? maybe pumping too fast? maybe you got the wrong size pulley on it?
Bill
