I guess you don't have the factory radiator anymore and it is much cheaper to go with an aluminum replacement but a stock one in good condition will cool a 400. A 51 GMC I built 30 years ago is still doing fine with a stock radiator cooling a 500 Cadillac engine with a/c and in our wonderfull 107 degree Texas heat. I did convert it from a low pressure radiator to a high pressure one that uses about (IIRC) a 16# cap.
