Spectrum fuel tank sender
 

I used o new one of these for the 81. The 3/8 and 1/4 return lines stick up 1/4" more than factory and do hit the trunk had to use thicker rubber cushion to mount it. It also has pretty cheesy looking plastic for the resistor /float. An exposed very thin wire with thin insulation runs to the resistor portion and is easy to scrape the insulation off.

I have a new in box GM one but sitting in the box for 30 years it does not have any ohms .

Try submerging the resistor can into some gasoline. Sometimes that's all they need to start functioning again.

Forgot to also mention new aftermarket one is shorter and will sit almost 1" higher in the tank than a GM one so you could run out of gas easier even with fuel in the tank.

I'll give the gas a try.

Take a soft new pencil eraser and lightly move it across the wound wire section, and take some 1000 grit (approx) and rub the under side the the arm that moves...

Hard to see inside the resistor part of the factory one you can see a flat piece of curved brass that slides inside.

I have not had mine apart for a long time to remember....

Brass contact rubs against resistor. Sometimes new ones have a protective coating which blocks the pieces from making contact. Try using a solvent to clean first, then if that doesn’t work, you may need to get more aggressive.

Fine Lines vent line is close to factory, but all of their lines have a different hose crimp on it and the fuel return line. New fuel return old vent.