A 6 volt starter will work fine on 12 volts, just don’t lay on it.

You need to provide only 6 volts, however you do it, to the fuel gauge, otherwise you will kill it, should be a colored tag on the terminal advising that fact, Jim Carter truck has that resistor for gauge. Resistor to feed heater, wipers, etc. Change bulbs.

There used to be a square ceramic resistor(about 4” x 4” square) no longer in production that had several taps, however they appear from time to time.

Ed

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.