When I converted my 40 to 12v I used a CVR (Constant Voltage Reducer from Ron Francis) to reduce the voltage to 6 V for the FUEL, TEMP, Oil gauges. In order to get the BAT. guage (which is really a volt gauge) to read accurately I isolated it from the other 3 gauges and used a 60 ohm reducer to reduce the incoming 12V to the BAT gauge to 6V. Read post #6 here.
https://www.fordbarn.com/forum/showt...olt+conversion