Gas gauge 74 Duster help

Judging from how you described it in the original post, I would say it is the sender. Here's why...

Typically, the gauge either works or it doesn't. All it is is a bimetal strip that heats up and expands a given amount, depending on the resistance it sees. The less resistance, the more current it draws and the more it expands (Volts = Current X Resistance).

The sender is a different issue. The sender is typically a wound bimetal spring connected on one end and open on the other. There is a set resistance through the length of that spring, but your wiper contacts it in different locations, depending on fuel level and position of the float. For instance,.... "Per the spec", completely empty with the float at the bottom and the contactor at the open end, there should be 80ohms across the contacts (empty). As you fill it with fluid, the float rises and the contactor wipes toward the connected end and resistance drops. Half tank should read approximately 25ohms of resistance. At full tank, the wiper should be as close to the connected end as it can go. Resistance should be about 10 ohms.

From what you described, and assuming it used to work,.... I'm thinking there is corrosion on the contactors so ohms never drop anywhere near 10 ohms, which is why you never go above 3/4 tank. Then, as you burn up fuel and the float drops, at somewhere around the half tank point, there is a bunch more crud or corrosion that it has become an open circuit, causing the needle to drop to empty. My guess is if you were to pull that sender out. You would find some crud on the wiper and/or spring, adding to resistance in the upper half of the tank, and a whole lot of crud/corrosion on the area the contactor rides when the tanks is half full or less.