While not an SMB, my '94 B350 project had a setup similar to what you describe. Here's a link to my build thread, page 8, wherein I grab seats from a junked '70s conversion van to install in my own. This was to be the dog's seat.
https://www.sportsmobileforum.com/fo...f-22667-8.html
The front seat swivels from the old van happened to bolt right into my '94 but the one for the rear would have required a little ingenuity. I simply rigged a wooden form to hold the seat base but would have drilled the metal floor and through-bolted the base eventually. Sold the van before completion.
Incidentally, the rear seat came with a lap belt and anchors built into the swivel base. Perhaps you'll be lucky enough to find something like it.
Good luck!