I think you have the right idea; letting the water circulate through the heater core would cool the engine more than the radiator alone. Turning on the heater fan on will help even more.

But if you have to leave the heater on in the summer to keep from over heating there is something wrong with the cooling system.

You only need to have water flow in the heater on when you want heat.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-