I just set up a passenger seat in my 1987 Bluebird. The driver's compartment is small, now that I constructed a wall just behind the seat. I bought a 'captain's' chair (a bucket seat from a used vehicle) cheaply. It has a base with holes in it so I placed the chair next to the driver's seat, drilled two 1/2" holes, making sure I didn't pierce anything underneath that I didn't want to, and bolted the seat down. It is quite sturdy, swivels, has a seat belt built-in, and I didn't have to do anything else. There's a little bit of climbing over it for the driver to get out, but since we plan on leaving the bus in one place for extended periods of time and do a minimum of driving, this seemed like the easiest and cheapest solution. Plus the seat comes out very easily to be used in my other bus.
Hope this helps.