I'm kinda having the same problem with a '53 GMC. I put a new sending unit in but the gauge reads full (tank is 1/2 full). I tried running a jumper to the unit and grounding the flange like baldeagle said but when I move the float, the gauge needle moves from over full down to full. I wait to hear if you resolved your problem and what you did. Someone told me the sending unit and gauge might be mismatched.
