The 2wd to 4wd conversion varies depending on the truck. I have NOT worked on a 1987 before. I have no idea if there were changes in the frames compared to the older trucks.

On the older models, the 2wd 1/2 ton trucks had a light duty and an optional heavy duty frame. The majority of the 2wd 1/2 ton trucks used the light duty frame. (These heavy duty 2wd 1/2 ton trucks used the heavier gauge frame rails like the 2wd 3/4 tons and 4x4 1/2 tons.) The light duty 2wd frames did NOT typically have the holes to mount the 4x4 cross member. A friend of mine had a 2wd 1/2 ton with the heavy duty frame. His frame had nearly all the holes to mount the 4x4 cross member. He knocked the rivets out and swapped cross members. It was a fair amount of work, but most everything lined up and worked properly. His truck had the small transmission tunnel. His solution was to keep the small tunnel for more interior room and add a body lift on the truck for transmission / transfer case clearance.

Bottom line: Look at the frame on the half ton to see if it has the mounting holes for the 4x4 cross member. If it does the swap MAY be feasible, if you don't mind doing a considerable amount of work. The other option is to swap the nice truck body onto a 4x4 frame.