I just don't see much point to the fan being on if the radiator and air flow across it are sufficient to do the job without the need of the fan. I mean it's not like you are going to put a 195 degree temp switch in the bottom of the radiator, it would obviously overheat. Something lower like 170 or 175 maybe, you'd have to play with the temp sensor to get one that kept the engine cool on heavy towing, but also didn't run the fan when it wasn't needed. To me if the sensor was in the top of the radiator it would turn the fan on as soon as the thermostat opened and it would stay on. That kind of defeats the point of a fan clutch. At that point you might as well just do away with it and run a regular mechanical fan without a clutch. Also if your coolant is low and the the probe is in the top chances are it won't turn on the fan. Have you ever seen an engine get air locked? Why does it take so long for the thermostat to open? It's because the air against the thermostat doesn't conduct heat as well as hot coolant would. Just my thoughts, take it or leave it.