Why not put one heater bung in each tube? One tube is under pressure, one is under suction…. Not as extreme as directly at the water pump, but you do have a pressure drop across the radiator…
… and you live in Southern California, where your heater shouldn’t get much use!
Not sure if you have considered it or not, but if you’re gonna track your car lot, they sometimes require positive shut off to the heater, from what I understand….
Some tracks, like INDE, don’t worry about that…
Rocky