1973 peterbilt 359 fuel gauge problems

Hey guys I'm working on a 1973 peterbilt 359.
The fuel gauge wasn't working got a new gauge and sending unit.
Old sending unit grounds through the tank and the old gauge has ignition and sending unit.
The new gauge has ignition, sending unit, and ground. The new sending unit has signal and ground. With everything hooked up as it was before the gauge reads half until you turn the key off. If the gauge touches a ground it reads all time. Iv tried switching wires around that either makes the gauge not work at all or full. Iv even tried using the old gauge and I get the same results. I'm at a lose anybody have an idea?