yeah, before anything i would just try grounding the sending wire and see if the gauge goes to full then you know if it's a issue before or after the sending unit... also sending units can lose their ground and the gauge won't work which is the most common cause of it, to the point they made this clip to jump over the rubber hose on the sending unit so it would have a ground to the metal fuel line.
Mine is reading, but I don't think it's right. I did have a good line jumper on it when assembled anyway... I think the actual fuel level is getting low now, so should get at it soon in case I decide to drop the tank...