Hi
I had the same problem fitting a factory bulkhead to my Citroen Relay which I got off another vehicle second hand. I removed the seatbelt covers and panel trim above on both sides, fitted the bulkhead, and then trimmed the plastic covers with a jigsaw. The first side i did took ages as I didn't want to remove too much. Once I got it right I was able to use the measurements on the other side and cut the panels to size quite quickly.
You can also use the piece of sandpaper on a block of wood to shape the cover to the bulkhead. If you do take off a little too much you can always run a bead of exterior frame sealer down it to finish it off, like the window frame boys do.
I also had to trim the roof panel back as well.
I would just make sure that your seatbelt operation works fine with the bulkhead in place before you cut the covers.
I figured that the van will be sold one day with the bulkhead so its no big deal to trim the covers.
Spruce