You cannot actually adjust the timing on your truck. You actually set cam retard and you need a scanner to tell where it is. The computer controls the timing, but the cam retard needs to be within a certain perimeter for it to set it correctly. Your spec is 0 degrees plus or minus 2 degrees. As I said though, You need a scanner to set it.
The 4.3 distributor can only go in one spot and can not be rotated. The cam retard is set by having a scanner and run the engine above 1,500 rpm when setting the it.