Instrument Voltage Regulator for 65 Dart

Your '65 Dart does use a push-in limiter, unless you've swapped in a Rallye dash...
Still, just go to RTE and get theirs.
RTE limiter - rte (rt-eng.com)
How to install them in a Rallye dash (or other dash where the limiter is internal to the fuel gauge):
RTE Gauge Faq - rte (rt-eng.com)