To build on what 86scotty said, one additional thing I've run into a few times was having to remove passenger seat belts (if there's captain's chairs in the back) from their stock rear locations in passenger vans in order to get the interior panel off so that I could get to something behind it. In your case you'll need to get behind whatever interior panel is in there so that you can get to the window clamp ring. The seat belts are mounted through factory trim and most conversion trim as well on the vans I've seen. The mounting bolt for the seat belt is a big Torx, but I can't remember if it's the same T55 plus that the front seats use. It's somewhere in there though, T50, T55, or T55 plus. It's big enough that if you don't have the right size, you'll have a tough time getting the trim panel off without it.
Also grab a box cutter or something with a longer blade so that in the event the window was sealed with a sealant instead of the compression gasket you'll be able to pry and cut, pry and cut. I always take a little 6" pry bar on junkyard runs and it always comes in handy for something. Good luck!