tortron is right hold up on replacing the radiator especially if it's an original factory brass unit they can be repaired or have a new core soldered in ~ try testing the sending unit in a pan of cold to increasingly hot water (maybe do it in the engine compartment with alligator clip ended ground wire see what the dash gage reads with the pan of boiling water)..?

My truck has two coolant sensors next to each other one for coolant temp to the dash gage & one for the ECM to control the engine (emissions?) Maybe your engine is not really over heating maybe the sensing unit is bad or even a bad gage..?

Anyway get a new fan clutch about $40.00 and keep posting your progress

Good Luck