I have one for the passenger side. I bought it for my 1999 Chevy from Sportsmobile when I had a pop top installed. It might fit your van. I sold my van 4 years ago, but still have the swivel seat base. To install it, you bolt it in place of the existing base. It uses the same bolts.
I didn't like it for 2 reasons. Its rotation was restricted by the puffy paneling on the door and it just didn't feel solid. I ended up putting the original base back in. If you flatten the door paneling, its rotation might be unrestricted.
If you come to So Cal, you're welcome to take it and try it. Because of its weight, it would cost a lot of $ to ship, but right now, it's just taking up space in a storage cabinet.