i agree. if you desire to save the box at all a 2 post lift would be needed. If you dont give a damn, yank it off with a tractor, if you have a front loader. Thats how i did my bed even though i have a 9,000lbs 2 post lift in the garage. I then backed out of the garage with the front loader and put the bed on stands, in the drive. I was able to do that without damaging anything.
I have moderately frightening ideas as to how to successfully get that off, if you dont mind damaging the box, without heavy machinery or a lift, but ill save that crazyness for a need to know basis. Hopefully you have the tools. Another very simple idea.
Again
toolstoolstools
...would be to cut it up piece by piece with a large acetalean (i dont think i spelled that right...lol) torch with a good cutting bit. cut the roof free, loose it, cut the tops of the sides off, then thebottoms, pull the pan it shouldnt be to heavy, in pieces
if you dont have these tools, like i said i can suggest a couple "shade tree mechanic" tips that might be per-sieved as nutty but will get the job done hahah
good luck