you wouldn't want a radiator fan sensor in your engine, it would always run when the motor got up to temp.
i used a short piece of exhaust pipe in my lower radiator hose. i welded half of a 3/8 pipe coupling in it for the fan sensor. also weld on a ground stud to ground the tubing and sensor.
my thoughts for using the lower hose? i figured to let the radiator do it's job cooling coolant, if it comes out too hot; then turn the fan on.
if you put in the thermostat housing or the upper hose, your fan will run as soon as the thermostat opens. even if there is ice in your radiator.