I'm in the middle of my build, but I have a pair of 3rd row seats from a full-size GM SUV (Expedition, Youkon, Tahoe, etc). Not sure which one... paid something like $100 for them on Craigslist.
Plan to build a steel frame that raises the seats 6 to 8 inches of of the floor, then build a platform behind just like what was described initially in this thread.
These seats have integrated seatbelts... Middle is a lapbelt but the sides have shoulder belts. They fold pretty darn flat and have extendable headrests.
They don't really have the width to go all the way across the van, but I should be able to work around that.... I am thinking about filling the space with an "arm rest"... the same height as the folded seat. I can probably get a cup holder in there and maybe a little storage cubby for the kids to use while we're driving.