How does the fuel sending sending unit sent the signal to fuel guage on a 71 dart.

You can measure the gage itself with a multi-meter and check against the factory resistance signals. Can you post a photo where it is leaking? Sounds like it might be the big rubber fill tube grommet ($12 ebay). Worst case is you will have to pull the gas tank, which is fairly easy if almost empty. Kill 2 birds.