Check sender with ohm meter. If good, probably gauge or ground issue. Gauge and sender are both 30 Ohms, use a reducer into gauge. I read that you may have old cloth wire and you may have connected new wire to old sender wire. When you rewire a truck you only use new wire, everywhere. NO old wire or old terminals. Eliminate all under cab, under body inline connectors. They are not needed for any reason. Direct wire. The only connectors you keep are the ones that connect directly to a component. Like headlights bulbs and some switches. Where practical, remove those also and use blade or ring tongue terminals. (or get new connectors). 70 year old connector internal contacts can be a real pain. I'm sure you have also seen the plastic connector shells disintegrate in your hands.
Most gauge/sender, switch, motor issues are better tested/troubleshoot on the bench. Isolates the components and identifies whether component or truck wiring.
