What is the pressure rating on your radiator cap?
My 216 uses a 4lb cap and I use a coolant reservoir like a modern car. This works perfectly , filling up the reservoir when hot and returning to the cold level after shutdown.
The gauge on the dash stays at 180 at all times. It did creep up to 185 or 190 during the hottest part of the summer.
Absolutely no overheating problems whatsoever.
