The new radiator is probably rated for far more pressure than the heater core can stand. A 55 1st. series probably came from the factory with a 4# cap. A good OEM heater core might handle a 7# cap, but a 70 year old one- - - -who knows?
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!