'09 was the last year of the 7' box supposedly....
The "cab" section on the 7' box Ranger is 6" longer than the 6' box one; the box section is 6" longer behind the rear wheels.
All boxes bolt in with 2 bolt at very front of box, 2 at very back and 2 ~15" forward of the back. To make a 7' box floor into a 6' box, you'd need to cut a 6'' strip out of the front of the floor (just back of the bolt holes) then weld the floor back together, then repeat at back (just ahead of the 15" forward bolts).
Doing the above also puts the underneath support rails in the correct position.
So, yes it could be done, but it would be a huge effort - butt welding 2 - 4' joints on the bed floor, priming and repainting.
Finding a 6' box should be easy - majority of Rangers - all SuperCabs, and Regular Cab short beds are all 6'.
Splash boxes floors are might actually be same size - all the difference being in fender panels. (I know they bolt up the same, but I've never measured the actualy sheetmetal).