Fuel Sending Outlet, size, for '70 Swinger

Diagnosis doesn't have to begin with jacking and crawling. The pal nuts that hold these gauges in the panel also attach them to a printed circuit board. Very often a loose nut is the culprit. Not to assume that be the case either. I'll assume the temp gauge works until informed otherwise. Anyway...
Pull the left kick panel, find the rear harness connection, separate it, attach blue wire coming from fuel gauge directly to ground, turn ign' switch on. If the instrument is good the needle will race toward full ( Keep this test brief so you don't fry the gauge ). If this proves true, jack up and crawl under is next.