If you just want to test the engine, get a couple of long flexible radiator hoses and take the radiator out of the equation by joining the two hoses with a piece of exhaust tubing with a brass inner tube valve stem soldered into it. Then, assuming the heater core isn't in the mix, go ahead and crank the pressure up. I've tested a race engine block and heads for cracks and seep leaks by plumbing a garden hose up that way and using city water main pressure. I found a tiny sand hole halfway down a cylinder. I installed a sleeve in that hole and saved a lot of expensive machine work.
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!