How old is the tank sender??
Might be worth slaving in a new sender unit and see if the gauge comes alive. One dodgy point is the earth lead for the sender - usually it's in the worlds worst place for collecting rust, if it isn't making good contact with the frame the sender won't send shite to the gauge.
There is one wire going front to back from gauge to sender - if memory serves correct it's pink. There's another wire coming from the sender forward to the crossmember just in front of the tank, you should see a termination eye screwed to the cross member - make sure you have good metal to metal contact here. I moved mine across a bit and made a new hole, removed the paint from under the eye end then covered both sides with some kick ass silicone type stuff.
Other than that make sure it's wired up correctly.
When I did my resto I used the old gauge with new wiring and sender.
At the end of the day it's a simple system - gauge, sender and wiring in between.
If you need a new sender unit you have to drop the tank
