[75gmck25]

Most of the 70’s GM vehicles I’ve worked with have the heater return connected to a fitting on the radiator, and most aftermarket GM radiators have this fitting. Some vehicles have the vacuum operated shutoff to stop flow through the radiator when the heat control is full cold, but many do not have the shutoff. I tried adding a shutoff to my truck and I still had relatively warm air in the vent position.

Bruce