troubleshoot fuel gauge

[70chevlow]

Ok so im having trouble with my fuel gauge . It partially worked before i had to cut the wires to install some seatbelts due to the previous owners choice to run both connectors through the original seat belt mounting hole. Any way the tank is not located in the cab it is under the bed. The previous owner had ran the ground from the sending unit all the way into the cab would it be better to cut it back closer to tank and ground it inside the frame rail? Next question i tried to do this and the gauge now does nothing when turning the key or when truck is started its not until you put it in gear and drive a little bit that the gauge begins to work but it moves all over the place what would cause this?