[70pontiaction]

To test, if I remember correctly, I could use a known good sender, attached at the tank or under the dash, but I don't have one of those.

If I disconnect the wiring from the back of the gauge, how would I go about testing and figuring out if it's a short, a bad sender or a bad gauge?

FYI: Gauge has never moved.

Another quick question: How many gallons or gas fit into a 1961 Ventura gas tank?

["QUICK-SILVER"]

A good test light and a volt/ohm meter should be all you need for testing.
Fuel guages work like an ohm meter, it simply reads resistance from the sending unit.
Grounding the sending unit wire is the same as 0 ohms/no resistance/unrestricted current flow, which is the same thing the sending unit is suppose to do when the fuel tank is empty. So grounding sending unit wire or sender terminal on back of guage should make the fuel guage show empty.
Higher resistance (90 ohms) is typical for a full reading on fuel guage. So bad sending unit, lost tank ground, broken sending unit wire, or bad/poor contact at wire terminals will make a fuel guage show full.

Anyhow, to get you started, applying ground to the sending unit wire should tell you if it's the sender or the guage.