66 Cuda Temp gauge issues

Sounds like a problem with the instrument voltage regulator. It regulates battery voltage to about 5V. It is likely gone bad, or the output is shorted to a 12V supply. Since work has been recently done, wiring may be the fault.

There are many posts if you "search" for instrument regulator. There are replacements available, low cost DIY using linear regulator, improved solid-state switch mode regulators, and the old style vibrator style.