Well,with info gathered from Crimson's U.S. military wheeled vehicles,and David Doyle's Standard catalog of U.S. Military Vehicles,It would seem that with the exception of one or a few,ALL of the CCKW tippers/dumps were long wheel base.
The dump bodies were built by Gar Wood or Heil with a movable partition to restrict the load to the rearmost part of the bed.
It also states that the earliest models(prior to 1942) were built with closed cabs,changing to the military open cab after that point.
Evidently lessons learned from having a bed large enough to overload the chassis led to the later M series dumps being all short wheel base.