Doing this at the moment.Not too bad a job easy-er if you've got van sides and no alpine windows.
Remove the roof and sides .
Decide how short you want it,and mark it up.
Drill out pop rivets drivers/passengers heads.
Remove sides and cut the rear half panel leaving an overlap (about 4 inches) You don't cut any of the front panel so you will be using the holes in that to pop the lap on to.A bit of sealant when you do.
You have to cut the gutter so a bit of careful cutting of the lap section is needed so that the gutters butt up. Pop it and weld the gutter.
Side panels cut from rear then i got some alloy angle and popped that on the rear corners.
Then cut rear door in half use top as lift up door with gas rams off an old hatchback and hinges on the top.I used normal front door hinges but ground them down so they are flat on the backs.