Take the wire that attaches to the sender at the tank. Use a screw or paperclip or something and ground that wire. At the dash, turn on key and gas gauge should peg 'full'. If it does, your wire to gauge, and gauge are fine, the problem is at the sender. When you say you ran a new ground, did you run the ground from the sender itself or just from the tank? As stated above, the senders get their ground via a short metal strap that clips to the sender fuel line and the fuel line on the chassis. Anyway, if grounding the sense wire does not cause the gauge to peg full, then your issue is likely the wire going from the tank forward. They usually go up through the trunk and then forward to the dash, so check the wire in the trunk area first, the rest is trial and error. Also, if none of your gauges work, check fuses first, then check things on the gauge cluster itself, like the dash voltage regulator. Basically, with a properly grounded, new sender you will just need to check everything else. Unfortunately, at 50+/- years old, the usual suspects are everything.