Regarding your gas gauge, you may now know this but there is no wire “directly” from/to the gauge. I replaced my harness a while ago but as I remember it is configured like this. The gauge is connected directly to the dash circuit board which then has a “trace” or circuit on the board which goes to the pin connector on the board which the main eng/dash harness plugs into. The wire for the gauge runs from there and is picked up at the connector plug from the dash harness to the rear harness and there should be a wire/connection point in the plug for the gauge in both.
The main service manual has a good schematic you can follow.
Also, if your harness is messed up you can get a new harness from M&H Elect Fab in Southern Calif. Mine had been unwrapped, cut, spliced and basically destroyed. I contacted them about 1-1.5 years ago and they had a B/P and made one up for me for my 62 Cutlass Convert. It is an eng/dash harness which is all one harness. Does not include the convert top harness or heater harness which are separate. They now consider it a “Catalog” harness which is important since they are so busy they are not building non-catalog or custom harnesses at this time. Lead time is 16-18 weeks I think.
Be very careful with the circuit board on the back of the dash pod. They are rarely available and have a problem with lifted and cracked/broken traces-both signal and ground. If gauges/lights aren’t working and all wiring seems to be good you may want to look there. It could be repaired CAREFULLY with trace repair or jumper wires.