Chevy electrical systems were negative ground, while GMC's of the same year had a positive ground system. The generator can be polarized to work either way. Original-equipment regulators had different part numbers for positive or negative ground polarity, while most replacement units are wired to work with either polarity. I second the motion to let the rebuilder adjust the generator and regulator as a matched set. Back in the 1950's, my father had contracts with several trucking companies to rebuild and calibrate generator/regulator pairs, which were swapped out on a time or mileage cycle for preventive maintenance. This practice virtually eliminated charging system-related road breakdowns.
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!