[19Fordy]

Rich,
If you are dealing with the OEM King-Seally gas tank sending unit and gauge they DO NOT operate on the ohms principle. Does you gas tank sending unit look like the one below? If not, no need to keep reading.
If so, it the OEM King-Seally unit used in Fords and Mercurys up to 1953 or 1954.
Here's what I would suggest.
Remove your gas tank sending unit from the tank and set it on a piece of cardboard in your trunk to insulate it from the metal floor. Then connect the wire from your dash gauge to the top center screw of your gas tank sending unit. Then, using an alligator clip connect a ground wire from one of the screw holes in your sending unit top to a good ground on your car, like the frame or floor trunk latch.

Switch ON the ignition and move the float thru its range of motion to see if it registers on the gauge. If it doesn't register, you may have a defective sending unit or defective gauge. Check to see if your float is full of gas due to a hairline crack. That is very common.

It is also possible to carefully pry off the top of the gas tank sending unit and clean the internal points with 600 grit emory paper. BE VERY CAREFUL NOT TO DAMAGE ANY INTERNAL WIRES OR THE CARBON RESISTOR. Use a bench vise to hold the sending unit without crushing it.

Hope this helps.