That's approaching the point of a blowup. Those old cores were not designed for high pressure and you're courting disaster with over 7 PSI or so. If you want a little leak insurance, run plain water in the radiator with a few ounces of Sodium Silicate (water glass) in it for several heat cycles. When I drove the fire truck with the big 502 GMC engine to KC last year for Ed Pruss, I sealed a minor head gasket leak with water glass. He told me recently that it's still holding, even after driving it on to Denver and adding antifreeze to it. I believe that system has a 4 or maybe a 7 pound pressure relief built into the top tank of the radiator.

That was a really interesting trip!
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!