I have a 1601. The fuel guage always shows half tank. I have filled it full and it still shows half tank. It is not stuck on half. When I turn the key off it will go to the empty mark.
Has it always done this? I ask becasue a tank with damage (dented/creased) will restrick your float on the sending unit assembly. Have fuel filter changes indicated brownish film on element and bowl?
The ballast float may have porosity from water/moisture and is partially filled with fuel and only raises part way (float sinking). Pinhole in solder or crimp seam - same results.
Or a hole in your filler neck screen has allowed trash to accumulate on float lift arm and prevent it from completing total lift.
You may want to remove dash and cowl just enough to expose tank top. Two wires and a few screws will allow you to pull sending unit and inspect. If nothing mechanically is hindering it's operation, I suspect the sending unit itself is bad.