OK- - - -you've got a real oddball there- - - -a "6 volt" high amperage alternator. It will probably produce around 7.5 volts when working against a fully charged 6V battery. Once you're sure the engine turns freely and there are no stuck valves or piston rings rusted to the cylinder walls, I'd suggest completely disconnecting the charging system and doing the initial startup on 12 volts. No point in dumping a bunch of money into an obsolete electrical system only to find out the engine isn't a good candidate for restoration. Most of the BIG GMC' sixes (413 to 503) got worn out and scrapped 50 years ago or more.
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!