fuel gauge problems

The limiter will affect the temp gauge as well. That is the key, "if they both" act up............

Some general ramblings, some of which you may already have done.

I try to encourage you guys to think of "the system" "end to end" "the path."

In this case "the path" is from the key---power up to the harness connector, harness pin to board traces, to the limiter spring contact on the board--through the limiter---back out another spring terminal and board trace---and feed to the temp and fuel gauge studs. Through the fuel gauge, out the sender stud, to the board trace, back to a harness connector pin, to the sender wire, down to the left kick panel connector, back on the tail harness, through the trunk, and down to the sender, through the sender, grounded through the fuel line clip, and "maybe" to body ground

You need to be suspicious of, and check all of these points, 'specially now that these girls are some 50 years old

Inspect the harness connector pins. If they appear loose or corroded, clean them up good use some ELECTRICALLY COMPATIBLE flux (not plumbing, etc, which is acid) and use electrically compatible solder to solder the pins to the traces.

Solder jumpers across the IVR contacts to the board traces. These, if you look, are just crimped into the board, and can loose contact.

The gauge studs can lose contact. Clean the copper around the board terminal, and loosen/ tighten the nut a few times to "scrub" the connection.

Rig the thing up with 12.6--14V power to the appropriate harness pin, and use resistors to check gauge accuracy. All three gauges (oil in a Ralley) essentially follow the same resistance

Suspect things like the push-on wire terminals at the temp and fuel sender. suspect grounding at the fuel sender.

Take nothing for granted. For example, one problem child turned out simply to be a bad power feed to the cluster. Enough to make the warning lamps glow, but intermittent and would not reliably operate the gauges accurately.

On my old 67 the harness pins some were broken, I simply soldered pigtail wires to the board traces and used a pair of "Molex" style connectors and got rid of the factory ones

CLUSTER GROUND. Clusters are "accidently" and poorly grounded by the mounting screws. Find a good common ground point on the cluster, install a pigtail wire, and bolt it to the cash frame/ column support.