Probably a 14 or 16 lb. cap. Every pound of cap pressure raises the boiling point 3 degrees F., so a 16 lb. cap will let the coolant get above 250 degrees before it boils. A clean radiator with enough swept area should keep the coolant below 230 in normal max-load operation, so you'd have a 20-degree safe zone before it boils. A 50% mix of antifreeze and water drops the freeze protection to -34 F., and raises the boiling point slightly, plus it helps the heat transfer, so don't run plain water, even in non-freezing weather.
Jerry
