A thermostat sets the minimum operating temperature- - - - - -the condition of the radiator and the volume of air across it determines how much hotter, if any, that the coolant will get once the thermostat opens. The only exception to that rule might be if the engine idles for a long period of time in very hot weather. Are you running an original radiator, or an aftermarket one that's rated for higher pressure? Ditto for the heater core. A 1955 truck probably had a 7 pound radiator cap that allowed the coolant to reach 233 degrees F. before it boils.
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!