Bob
I agree with you, as you suggest, a tractor can't count on its velocity to get air through the radiator. And some use push and some pull to move the air with the fan blades.
Also, as far as the compelling argument you bring up about the front blade, Bird addressed support of that in his 12/30/01 8:58 pm message in this thread. And I would weigh in on the side that a front blade makes no difference. Debris in the radiator, and load on the engine would tend to contribute to a problem with overheating way before anything else. Probably the same thing with trucks with front blades overheating. Its likely not the blade restricting air, but maybe snow in the radiator blocking flow, or keeping the fan blade from turning after punching through a big drift. Have had both of those things happen. (And when the front end is fully packed with snow, don't whip out the electric heater to melt it quickly. It will make a twisted mess of the plastic covering on the bumper and the valance /w3tcompact/icons/hmm.gif).