help needed: want to fix the gas gauge once and for all

You need to think of the gauges all the way from end to end, as a system, and just about every component and connection can have a pitfall

POWER. Power to the cluster comes from the "IGN 1" or "ignition run" wire to the harness connector and the pins on the board. THESE PINS can be loose, dirty, broken, or the harness connector the same. Of course you actually have to HAVE good solid battery voltage.

GROUND The gauge VR and lighting circuit needs a ground. As old as these cars are, it's good to hook up a separate ground pigtail to the board ground screw, and run it to a nut/ bolt/ star washer on the dash lip or column support

PC BOARD. ON my 67, some of the connector pins were broke and loose. In my case, they were so bad, I bought two Molex style connectors from Rad Shack and wired pigtails to the board traces. In any case, inspect, clean, repair the harness connector

The VR socket on mine is brass spring fingers which were NOT making contact with the board traces. I had to solder jumpers from the contact fingers to the board

VOLTAGE LIMITER. It can be bad or NOT ACCURATE. Do yourself a favor and buy a good modern solid state replacement

On mine at least a couple of the gauge studs were not making contact with the board. Loosen/ tighten the nuts a few times, or replace them with "real" nuts.

GAUGES Can be inaccurate. HOW old are they, again?

I gambled that both of mine would not be the "same amount off." You can compare them using 1.5V alkaline batteries

See this thread:

http://www.forabodiesonly.com/mopar/showthread.php?t=205270&highlight=gauges

CONNECTORS AND WIRING

The temp gauge goes through the bulkhead connector, a source of trouble

The fuel gauge goes through the kick panel connector to the rear harness, not very common, but can become corroded

BOTH gauges can have troubles with the original molded connectors, breaking internally, loose, corroded.

The fuel sender MUST have a good ground. If in doubt, and you can't do anything else, use a small hose clamp, scrape the fuel outlet clean, and wrap a wire around it several times, then put the hose clamp over that. Ground to the body.

SENDER ACCURACY

I made my own sender/ gauge tester, but you don't have to:

http://www.forabodiesonly.com/mopar/showthread.php?t=179517

If you do nothing else, go to Rad Shack and buy 4 -- 100 ohm, 1/2 watt resistors and put them all in parallel. You can substitute this combination for either sender, and after allowing the gauge to fully stablilize, either the fuel or temp should give you a 1/2 scale indication.

You can buy other resistances to use, they are:

L = 73.7 Ohms (empty)
M = 23.0 Ohms (1/2)
H = 10.2 Ohms (full)

If you have an accurate ohm--meter, you can check your fuel sender directly. Run the tank to near empty (carry a spare can?), then park it level, and drain out the pickup with a hose until fuel starts to spit or quits. In the chart above, the sender should show about 73 ohms. One source I found lists a little higher, 78 ohms, but there is some leeway.

'S 'bout all I can think of fer now, except it seems to be a agreement that all repop senders are crap.