Martin is correct (as he most always is). If all they did was convert YOUR 6 volt starter, them all is well. If they sold you another starter, and it being a 12 volt, then there's the issue. You need to know if what they gave you was actually your old starter.....or another starter.
Side by side, laying on the workbench, a 6 volt will look exactly like a 12 volt starter. The bendix (the starter drive) is different on the 12 volt. (I think the bendix shaft and tooth count on a 12 volt is larger....I may need correction on that). But the later model trucks, that came with and are using a 12 volt factory design, have a larger flywheel (which makes it a larger tooth count flywheel).
