Have the heater core replaced with a higher pressure rated one, run the modern radiator, and you'll be able to run a pressure cap in the 14-17 PSI range without a problem. That will eliminate any coolant loss whatsoever, particularly if you also run a small recovery tank. A 15 lb. cap raises the boiling point of the coolant to 257 degrees, far higher than a stovebolt engine ever gets during normal running. The water jacket and cylinder head handles the pressure easily- - - -the heater core is the limiting factor.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!