The flywheel diameter where the ring gear presses on is a different size between 6 and 12 volt, so you will need to use a 6 volt flywheel if you want to use a 6 volt starter and a 12 volt flywheel with a 12 volt starter. 1955-59 trucks used a foot starter if you want to use a 12 volt set-up, although a 6 volt starter works fine on 12 volts.
As far as flywheel diameters between 9" and 11" clutch the outside diameter is the same the only difference is where the flywheel is drilled for the pressure plate.
