When I rebuilt a 53 Chev wagon I took the parts from a 55 Chev for the swap (Starter, Generator, Coil, bulbs, Heater Fan motor, etc). Many folks told me to just leave the 6 volt starter on but it burns out after a while and you need to get a new one. I seem to remember someone saying that the tank send unit on these acts as a variable resistor to give you the action on the guage and when I changed over to 12v I dropped a 12v send unit in and seemed to have ok action on the guage. Perhaps the car unit is designed with a different resistance than the truck to begin with? I remember being amazed that I didn't have to put in a step down resistor. I sold the car a couple years later, so don't have it handy to check on longevity of this set up. All other guages functioned as normal. Hope this is of some use.
