[rotorwrench]

The bung designates cap fit by the depth of the bung, the diameter of the spring seal seat, and the outer diameter of the bayonet connection with the cap. It's likely a safe bet that a radiator such as Griffin's can take the pressures of a modern radiator which would be closer to 14-psi but a person would have to confirm that with Griffin. If the Griffin radiator was designed to take a 41A-8100-A or later revision cap then it should take the highest pressure available.

I don't think the 1939 Fords had a pressure cap originally although I believe that the 41A-8100-A cap, & later versions, (perhaps made during the war or later) would fit radiators back to 1938. The early ones just had a cap with a rubber seal and no pressure spring seal. Most folks agree that the early radiators were not meant to take much more than 4 psi of pressure and even less after 80-years or so.

If the head gasket is leaking, I'd suggest removing the head and replacing it. Flatheads need all the help they can get in cooling the engine and any leak is just going to make that situation worse.