300 series would indeed be considered a 1 1/2 ton. A 280 series (which is a "Rare" truck,you don't see that many) would be a 1 1/4 ton. 350 series is a 2 ton, 400 series 2 1/2 ton, 450 series 3 ton, & 470 series 3 1/2 ton.
Spanky Hardy Collector Of Fine Old G.M. COE Trucks & Antique Holmes Wreckers