If you look at ebay for a PMA (permanent magnet alternator) coil you will see several stator coils listed and they are not too expensive. Of course for this application you would not use a permanent magnet rotor as you want to be able to turn it off. The point is that some of these coils can put out well over 300 volts and a stock rotor can handle well over 12 v for short periods of time. The one thing to watch is the diodes, use the style alternator that uses the press fit ones on the plate inside the back as these can be easily replaced with 600 volt ones. I think that would be a 10si or 12si early model GM alternator.