I think I understand I work in the hvac business and we use what is called a backdraft damper on an exhasut fan that opens when the exhaust fan is running then closes when the air flow to the exhaust fan shuts down. I had thought about using some sort of flap or damper or something that was gravity operated but when the air flow from the road exceeded the tension of the flaps then the would open to allow air to the radiator.

Oh yeah, I am pushing the air through the radiator from the front because I have 3/4" between the waterpump pulley and the back of the radiator.

1951 GMC 1-Ton Flatbed Dually
1946 Chevy 1/2-ton (belongs to my Father)
1951 Build Progress Album
1951 GMC video