Hello Mike, it may help to really, really clean the ground wire bolting point at the tank. Must be bare metal and all the terminals must be bare metal and the wire not broken.
Then be sure your gauge frame is grounded to dash. It's usually just the ground path that is intermittent. To test this theory, Remove sender, run a test wire from battery ground post to the sender flange grounding tab/wire. Just run it thru the cab window. Direct to sender. Keep the sender gauge wire connected to sender and see if the gauge starts working. You can move the float up and down by hand and watch gauge. Key on. Report progress.
I assume you have the ground wire between the sender flange tab and the cab sheet metal? Usually this wire is already welded to flange on 60-66 sender....or there may be a tab for a spade terminal. The wire must be screwed to a bright bare metal spot on cab metal. No paint, no rust, no corrosion, no washer in between. Good tight crimped terminal if applicable.
I believe the gauge is grounded by the round brass slug on the back touching the recessed can, can screwed to back plate and cluster screwed to dash. Would not hurt to add a ground wire somewhere or test gauge ground with a direct jumper to battery. If other gauges and dash bulbs work, it would seem that you do have cluster ground. There are dozens of places to lose ground on a truck. Many of the ground paths are just screwing or bolting pieces together. If you live in a moist area, corrosion sets in.
