A modern aluminum radiator should be able to handle 15 PSI or more without a problem. However- - - -the heater core might be another matter. I'd strongly advise getting the heater rebuilt with a high pressure capable core, and installing a 15-17 PSI radiator cap and a coolant puke tank. Then don't get concerned about coolant temperature unless you see lots of overflow. Anything below 230 degrees is normal.
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!