could this be causing an issue?

The test resistors go from the sender wire to ground. You can start with temp as this is of course easy to get to under the hood. Hook the resistor from sender wire to ground, turn on the key, and leave it for a minute. If the temp gauge is way off, I'd assume you either have wiring troubles and or a bad instrument regulator in addition to fuel guage accuracy or damage.

On my 67, mine was a mess, and had several problems, typical of ANY of these

1--The pins on the PC board for the harness connector were loose or broken.

2--The fingers which form the socket for the limiter were not making good contact with the board, and needed jumpers soldered across.

3--The limiter itself was very "iffy" and I replaced it

4--On both gauges the "fake" tinnerman nuts were not making contact at the gauge studs, which must make contact with the PC board.

5--In addition, the wiring/ harness connections in the sender wires can be intermittent

6--Senders can be bad or intermittent.

I keep "preaching" LOL to guys to think of their gauges as a SYSTEM, and end to end system in one big package.

By checking with test resistors, OR checking resistance of one or the other and knowing what to expect, you can get an idea of how accurate things are.

This is supposed to be close to the resistances at empty (cold) -- 1/2 -- and full (hot)

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