I've rebuilt and tested a 6V generator and it isn't that hard. Instructions are in the shop manual. Here's the instructions for testing a generator on the bench.
I'm not sure you could even swap out the field coils from the 12V to 6V generator, and that would assume they're good to start with. Just go with the 12V generator after you test it and confirm it's OK.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.