It's impossible to put a 12 volt armature in a 6 volt housing..
I'm surprised you'd say that Jerry. For someone with your mechanical skills, I'd say that nothing is impossible.

I've not pulled a starter (either 6V or 12V) apart, but would it be possible to use the end plates from the 12V along with the armature? Just asking out of ignorance.
But the whole 6V to 12V starter conversion may not be worth the effort, what with the drive end differences as well.