Like Rick and Paul have said, the car center sections have a different bolt pattern than the trucks.
The trucks are C-clip style axles as well. So even if you could put the car style center section in, you would not have any means to hold the axles in.
After looking at my Hollander and Chevy parts books some more last night, I came to the conclusion that the 58-62 truck posi rearends were an entirely different animal, as Rick has explained. If you really want posi, you will want to switch to a different rearend alltogether, I would think. Some have used full size Blazer rearends, also some import pickups. 6 lugs is 6 lugs...
