Measure the wheelbase to determine if its a 3/4- or 1-ton.
GMC offered 1/2-ton trucks in both short and long wheelbase. So a 1/2-ton GMC could have either 116 or 125 inch wheelbase. Chevy did not sell long wheelbase 1/2-tons, but GMC did.
A 3/4-ton or 1-ton will have 8-lug hubs while the 1/2-ton will have 6-lug.
So if it has 8 lugs and a 125 inch wheelbase, its a 3/4-ton. If it has 8 lugs and a 137 inch wheelbase its a 1-ton.
If it really is a 1-ton you might want to trade it off on another because lots of people want that long box 1-ton pickup.
Once you know the wheelbase you can look for an S-10 that is close to a match.
Scott is right, you are giving up your load rating if you swap to a S-10 frame and suspension.
