You are not thinking this correctly. The only thing that matters is percentage of power. Two identical aircraft one with fixed pitch the other with a constant speed flying at the same climb or cruise speed at 75% power will be burning the same amount of fuel and loading the cooling system the same. Only difference will be the rpm on the constant speed may be lower, or it could be the same, the pilot has that control.
The rpm of the prop will not effect cooling air flow, only air speed matters, if both aircraft are at 75% power airspeed and therefore the cooling will be the same.