[JSeery]

I would try and go back to the original sending unit if possible, the resistance unit will be difficult to get to work (they can be made to work after a fashion, but not easy). Grounding the sending unit wire should give you a full reading on the gauge, so that seems to be working ok. Might try grounding one of the sending unit mounting bolts to a know good ground.

What the original sending unit is doing is opening and closing a set of point that is related to the amount of pressure applied by float. So if the extra ground doesn't help, check and see if the float is working correctly. If that is not the issue, you may need to open it up and see if the points need cleaned.