Sorry, I didn't realize you were dealing with an earlier car.
If this is the gauge system without the voltage stabilizer (skinny needle that can bounce all over the place) the resistance values are different from the later bimetallic gauges I mentioned above and the system is much more finicky.
For the magnetic gauges (w/o the stabilizer) the resistance range goes from nominally 10 Ohms = Empty to 80-90 Ohms = Full. This direction of resistance change is opposite that of the later bimetallic gauges I talked about in my first post. For a great explanation of that gauge system and how to tweak its calibration visit Barney Gaylord's MGA Guru website.
However, I do want to mention that with the magnetic gauges the fuel gauge itself MUST have a good ground connection or it won't read correctly. If during your work you disconnect any wires from the magnetic gauges note where each wire was attached. Unlike the bimetallic gauges, the wires have to be on the correct terminals (green to the gauge's "B" terminal, green/black to the gauge's "T" terminal... B for battery, T for tank). If you have a break inside the sending unit, if the tank or sending unit loses its ground, or if the green/black wire is cut/broken somewhere the gauge will read "beyond full"... not empty as you might expect.