A 6V generator won't last long (if at all) trying to make 12 volts. 12V Delco-Remy generators have red painted ID tags- - - -6V units have black ones. The internal wiring of both armatures and field coils is different. There are also different amperage ratings for generators- - - -and the regulators might have to be adjusted to match up with the generator capacity. Don't "assume" you can just bolt up mismatched parts and expect them to play well together. Drop me a PM and I'll be glad to help you with the conversion- - - -I've been working with generator/regulator systems since the early1960s, as well as spending 30+ years teaching automotive electricity.
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
Love your enemies and drive 'em nuts!