What temperature are you trying to maintain? Unless boiling coolant is overpowering the pressure cap and spewing out of the overflow, the engine IS NOT overheating. A 15 pound radiator cap will keep the coolant liquid up to 257 degrees F., and I seriously doubt you're running that hot unless there's something very wrong with the engine like a cracked head or a blown head gasket. Is the fan and the water pump turning the right direction? Most engines that came from the factory with a serpentine belt system use a reverse rotation fan and water pump. Normal temp can be a minimum of what the thermostat opening point is, to 20-30 degrees or more above thermostat temp when idling in slow traffic, or running air conditioning, etc. It looks like you might be chasing a nonexistent problem. A 350 engine block and cylinder heads alone has a coolant capacity of well over 2 gallons, almost 3, not counting the volume in the radiator.
I have solved real overheating problems, not imaginary ones, during trailer towing with my 59 Suburban, by installing a cross flow radiator specified for a 454 motorhome engine, and making it fit the radiator support.
Jerry
