Use a rubber expansion-type freeze plug that will fit the radiator neck hole below the overflow tube, and center-drill the bolt to accept a short length of rubber tubing and a Schrader valve from an old inner tube or a tubeless tire valve. Then use a bicycle pump to put a little pressure on the system- - - -maybe 3 or 4 PSI. DO NOT use an air chuck- - - -you'll blow out the radiator core or the heater.
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!