Working in no particular order now.
I bought a set of chairs on craigslist for $400 (too much, I think). The set included four chairs and a bench seat out of a van or something. The bench seat was in pretty bad shape, but the four chairs were good enough for me to work with and I put together two good front seats for the conversion. I think I can sell the remaining seats as well as the original driver's seat to offset the cost.
The set came with a bunch of pedestal parts that weren't exactly what I needed, but I was able to cobble together the parts for two really nice swivel pedestals. They even have a cool quick-disconnect feature so I can move the chairs out of the way while I work inside without having to get under the bus to unbolt the bases.
Eventually I'll have a tabletop that can be temporarily installed between the chairs for dining. There is more legroom between these chairs than it appears in this shot—they slide outboard a few more inches. Besides, it's mostly going to be me alone eating, so what do I care if it's a little tight under the table?
This is the pedestal details. The seat belts attach to the solid bar welded to the back edge of the swivel plate:
I added a three-inch steel channel to give me a little more height. I couldn't comfortably see the instruments in the original seat and these were far too low. The height is perfect now.
Picture of the locking arm on the pedestal base. There is a key underneath the pedestal that goes into the cammed hole on the base plate. A 90-degree turn (with help from a hammer, hence the worn rubber sleeve) locks it down.