The M1009 is the only truck in the CUCV series authorized pioneer tools according to TM9-2320-289-10, although there is no provision nor stowage instructions in the manual for a pioneer rack to be mounted. I never saw one attached to any of the CUCVs I saw in the service.
A couple of suggestions are to mount the rack to the inside of the tailgate, reinforce the rear seatback and mount the rack there, or, as mentioned, mount it against the window. Just remember, in case of accident, any such tools need to be VERY secure or they become missiles bouncing off the walls and passengers.
The pic is of the Suburban M1007-clone and the rack he mounted.