A '54 came with a 6V starter. If it's still that way after 52 years the starter will work in your '48 fire truck. By you saying you know how to wire it I'm assuming you already know you'll need a push button switch for it because it has a solenoid rather than a mechanical (foot operated) engagement mechanism.
The 6 volt starters (and flywheels) have less teeth than a factory 12v starter and flywheel.
True about the flywheels but the starters both have 9 teeth. The teeth are shaped different to mesh with the proper flywheel. Side by side the 6V teeth look fatter.
