Most of the replacement heater cores I've seen are also rated for high pressure, but with a clean OEM size radiator, there's no need to build a pressure cooker. The only reason to have a 15# cap is to raise the boiling point of the coolant to 250+ degrees F. That lets today's car manufacturers build radiators half the size they really need to be.
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!