What size/series "big bolt" and what size/cylinders engine?

GMCs with 6 cylinder engines stayed 6v (positive ground) through 1956 (standard equipment). 400-500 series trucks had an optional 12v system in 1956. All trucks with V-8 engines in 1956 had a standard 12v electrical system.

All GMCs had standard 12v negative ground from 1956 onwards.