66 Cuda Temp gauge issues

Yes, Voltage limiter in that gauge assembly is inside the fuel gauge and feeds the temp gauge. when it goes bad in your case it is sending batt voltage to the gauges.

you will need new or rebuild fuel gauge, and add an updated external Volt limiter. for now disconnect the temp gauge at the sending unit so as not to burn the new gauge up if not already.
contact Ben at http://www.demonivr.com/ He is a member here and extremely helpful in these matters. He can rebuild your gauge(s), calibrate, and set you up with a new external VL.