First thing you need to find out is what you have for a flywheel as this will determine what to use for a starter. They could have used either one when they retrofitted the newer engine. As said allready, the 6 volt system has a different tooth count from the 12 volt, but both flywheels/starters can be used with your engine/tranny combo. There are numbers on the engine side of the flywheel that can also tell you what it is if you don't fell like counting teeth.
As said before, 55-59 or so trucks will have the push start 12 volt starter if you need one. Otherwise, the 6 volt one can be used on 12 volts. I had my '53 setup this way for years without problems. If you want to use a solinoid starter, changing the ignition switch isn't necessary. The factory did this as an option in both 6 and 12 volt versions and the wiring is shown in the factory shop manual. All you need is one of those cheap clamp-on horn buttons to do it. Turn key on, hit the button. My Dad's '49 car was that way from the factory.
