I'll try to give the basic ideas of a homemade fix, but I haven't actually done this myself...
The stock Ford sending unit uses creates between 78 and 10 ohms resistance based on the level of the fuel in the tank. The factory design uses a printed circuit to create this resistance, but over time, this circuit becomes worn and might read accurately for part of the tank and then empty for the rest, or just read empty all of the time.
To fix it, you would need to acquire some resistive wire, and measure out a length that is in the neighborhood of 80 ohms. The best place to get it would be scavenging it off of a wire wound potentiometer. My problem was find a wire wound potentiometer that was low enough resistance (which equates to long enough wire to wrap around the frame) You then need to wrap it around a homemade frame that would take the place of the printed, duplicating the sweep of the circuit and attach the wire to the leads where the circuit was connected. Reassemble the unit, and plug it in to the truck and test the full range, making sure that there a no gaps in the readout, and if satisfactory, repeat the process to the other sending unit.
Unlike the stock design, this will not wear out any time soon.
If I end up re-doing mine, I'll try to make a writeup of it...