It depends on the design of the gauge. The 2-coil system your friend is talking about is called a "balancing coil" gauge, and it will probably work OK on a wide range of voltage. One coil has a constant current flow, and the other one is variable according to the position of the tank float. The differential magnetism on the needle moves it from empty to full based on the variable magnetic signal. Late 50's and newer Ford systems used a 12-volt to 5-volt regulator to power the gas and temp gauges, and used an idiot light for oil pressure. The gauges could be made more sensitive by wiring them to work on the reduced voltage. Not sure which type of gauge system the stovebolt trucks used.
Jerry

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