Instrument Voltage Regulator for 65 Dart

There's an old old "how to" on this. You mount it externally, (like a plug in) and wire it up to the posts at the fuel gauge, open up the fuel gauge and CAREFULLY bend and RTV the contacts so that they cannot close. Inspect the heating element and investigate cutting that loose

RTE Gauge Faq - rte

HOW THEY work. The 3 terminal early fuel gauge, and the later Ralleye clusters, one terminal of the fuel gauge is connected to switched 12V and feeds the INTERNAL limiter. This terminal is then used to feed the new, EXternal limiter. One terminal was used to "out" jumper limiter controlled power to the other gauges, and now you will use it to feed the output of the new limiter to all three gauges. The third terminal is of course the sender

I myself have never done this (as I don't have one of these cars) but I've read plenty and what Redfish has posted on here. It is extremely important you are careful "in there" and that you get the old limiter safely isolated so it can not cause damage and also of course that you do not damage the fuel gauge with your surgery