I have run 4 different 6 volt trucks converted to 12 volts for a total of 100,000 miles combined over a 35 year span without burning out a "6 volt" gas gauge. I don't think jump starting a 6 volt truck with twelve volts will hurt anything. Just don't turn on any lights or the radio while doing it.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission