how many lug are the axles? 6 lug 1/2 ton, 8 lug 3/4 and 1 ton.
the gm heritage site shows the difference between 1/2 and 3/4 and 1 tons are wheelbase in addition to capacity, so all 116 wheelbase are 1/2 ton (3100) and all 125.25" wheelbase are 3/4 ton, and all 137 wheelbase are 1 ton.
so without some other evidence like your truck having 6 lug axles, I would say its a 3/4 or 1 ton depending on the wheelbase.