sounds like yer tryin to hook electricity to a mechanical gauge sender!!
the 'hard thick line' if you look at the ends is it solidly crimped right into the threaded sender part and same into the gauge? [like not a clip-on or screw-on electrical connection]? called a capillary tube, and those gauges do go bad, and don't respond to shock treatments!
many/most of the 55's still had mechanical temp gauges, so if you've got onea those, the whole thing needs to be replaced as a unit [gauge/sender] - an elec gauge has wire connectors on the gauge and a separate sender that you can get at NAPCO
the gas gauge will need the proper sender resistance to work the gauge, but if you've got the hot side of the gauge hooked up, grounding and un-grounding the other side should make it read empty/full so you know if it works
yer mileage may vary! others might have other suggestions
Bill
