The main difference in the 6 volt and 12 volt starter armature is the diameter. I believe the 6V is bigger, so the 12V starter drive won't fit at all. You mention that the engine is from a 12 volt car with an automatic trans.- - - - -if you've still got the stomp start, I'll bet somebody re-used the 6 volt flywheel, starter, and bellhousing. If you've got a key start, it could have either 6 or 12 volt, as the pre-1955 powerglide cars had key start 6 volt setups.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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