Since you refer to it as a Gauge, and describe the sending unit as bell shaped, it sounds like you are describing the part in the picture below. I believe that they are the same from 1965 to 1985 in Ford cars and trucks that use a gauge.
A simple test would be to have an assistant sit in the car with the ignition switch on, watching the gauge, while you provide a ground to the lead that attaches to the sending unit (Just a few seconds is enough). If the gauge needle moves, the sending unit is likely bad. If nothing happens, the problem is in the gauge, or the wiring to it.