The finned item under the hood is a selenium stack rectifier- - - -it turns the AC current produced by that big alternator into DC to charge the battery or batteries- - - -usually more than one for a rig that size. Are you sure that's a 6 volt system? Most alternator equipped vehicles were 12V even back then.

There should be a label somewhere on the alternator indicating voltage, and possibly ground polarity. Most GMCs in the early 1950's were positive ground.
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!