The ground polarity doesn't matter. If cables and/or a starter smokes, there's a dead short somewhere. Start from scratch with a new starter (rebuilt, actually). New ones haven't been available for over 40 years. GMC trucks run positive ground- - - - -Chevy is negative. 8 volt batteries were used in tractors- - - -never on street vehicles. By the time the charging system gets modified to accommodate an 8 volt, you could have bought 2 or 3 correct 6 volt batteries with the same money.
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!