If the trucks was ever driven with this electrical system, the light bulbs are the best way to determine 6v vs 12v (if you can find/read bulb markings).
Not all 12v ignition systems used the external ballast resistor. Generators, starters, and coils quite often have no markings.
Yes, a few people have reported using a 6v fuel gauge in a 12v system without using a voltage reducers. Far more people report ruining their 6v gauge (or finding a useless gauge) in their converted 12v system.
