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

I could go through my achives and find pics to prove what I think is wrong with your fuel sender. You could ask Mullinax95 to confirm what I'm saying..
The sender has a moving contact that scrubs a risistor winding to create the different ohm values. Eventually the scrubbers wears through so the end becomes much like a fish hook. once it catches the resistor wire, thats the only value it can send.
One other common fault is a saturated float.
Either failt equals new sender.
I'll go a little further and predict the sender you remove will be shaped different from the replacement you purchase. Both designs work and both have their flaws. Nothing lasts forever. Hope this helps