Charge wire mod fuel gauge question.

Original ammeters are a high amperage series instrument, and are completely a separate circuit

I believe "the old girls" had the gauge regulator/ limiter IN the fuel gauge. That operates fuel/ temp and oil gauge if installed. One problem with some years is that the clusters get grounded "accidently" via mounting screws.

If the limiter is internal, your fuel gauge will have 3 studs. One is switched 12V into the unit to feed the limiter. One is the fuel sender. The remaining one is the regulator output which is then fed to the temp/ oil gauge.

This site has some info on replacing the original limiter/ regulator with an external mount one. You have to block the contacts or otherwise disable the original

https://www.rt-eng.com/index.php/RTE_Limiter2_Faq.html