65 Barracuda gauge problems

That's why we like photos. Your rallye cluster (w/ tachometer) is totally different than my 65 Dart's. If the rectangular can Vreg is factory, then I am even more confused. I thought those only started ~1967 in regular clusters and not until ~1972 in rallye clusters. However, your fuel gage connectors look totally different than mine, so the Vreg was probably not in there.

First, get a multimeter and measure the Vreg output. If you decide to replace it, I suggest getting an electronic one, which are affordable and much better. Yours looks like an original "thermo-electric" type that pulses on and off to give an average 5 V output.