Hy Joe Finkelstine, we are having this same discussion about flywheel pitches (tooth count) and starter drives elsewhere on this site. The flywheels tooth count 139 for six volt flywheels must be matched by a starter with a 14mm armature shaft which will have the correct pitch on the starter drive to match the flywheel. The higher tooth count on the twelve volt flywheel (sorry I can't remember the exact numbers 160 something) must be matched to a starter with a 12mm armature shaft that will have a starter drive with the correct pitch for that flywheel. A six volt starter will survive quite well operated on twelve volts, providing that the engine is maintained in a good state of tune and excessive cranking is not necessary. My only concern is that you mentioned the shop said the starter on your truck is from a car and should therefor have a solenoid on it for activation, I'm not sure how the solenoid will be effected by the twelve volts.Hope that clarifies.