I believe the stock wheels on a ½ ton would have been 15x5.5 or 15x6 on some models. Here’s an earlier thread that shows some oem specs for various models. I know there are other threads like this but didn’t find them on a quick search:
http://67-72chevytrucks.com/vboard/s...t=stock+wheels
Split rims were generally used on heavier trucks with multi-ply truck tires. By design the main part of the wheel has a center mounting disc, the hoop the tire fits over, and a rear flange for the tire bead to rest against. A separate ring with a split in the center goes around the face of the wheel and holds the assembly together. These were very common on big trucks up until the 1970’s, their primary benefit the ease of tire installation - especially on big heavy truck tires back in the days when this installation process was done manually with tire spoons.
Some time in the 1970’s there was a rash of fatal accidents when bent or damaged rings on big rigs blew off the truck and killed pedestrians or passengers in nearby vehicles. The same risk was prevalent for anyone mounting these wheels. If they weren't inflated inside a safety case, the ring could be blown off with explosive force causing serious or fatal injury. Popularity dropped to zero, to where it is now almost impossible to find anyone that will work on these wheels. Hence the term widow maker!