Yes, your original 6 volt generator can be converted to 12 volt, however, you have to change the armature and field coils and voltage regulator. You cant just swap out the fields out of a later model 12 volt generator. The pole shoes, which are what we call the two parts that hold the coils in, have to be changed as well and then there is a matter of some wiring to contend with internally. There never was a 12 volt conversion coil for the older Chevy generators, thats why you have to change the coils and pole shoes.....the pole shoes have to match the ID of the coils etc. This is what you have to do to convert the original generator. If your not worried about the original unit then you could probably go with a complete later model generator but it will have to be a long case and not short because of the mtg. bracket. I do this all the time and if you need more info feel free to PM me and I will forward you my info.
Thanks
Ken