Edited: another fuel/temp gauge thread, but turns out the harness is melted together

I have seen them available at autoparts stores.

Classic has them

IVR607 - 1966-74 Mopar A, B & E-Body Instrument Voltage Regulator

RT Engineering has the solid state one.
It is expensive but I like the protections it offers.

RTE limiter - rte

If you buy an IVR and the underside looks like this with the little screw on the left just above the left terminal it is a mechanical type IVR