I DID THAT! yep… I took the back seat from a very old (first gen dodge) minivan which has integrated seat belts (3) and mounted it sideways along the wall behind the steps. I mounted the seat base to a sturdy wooden base, then bolted the base to the deck of the bus. I did it this way because it's not meant to be bolted down, but latch-removeable… I had to engineer a way to fix the frame to the wooden base using d-rings looped over the latch hooks in the back, then thru-bolts securing the front frame brackets. (still with me?) That left me with a seat mounted to a base that I could move into place and EASILY mount the base to my deck… there was simply no way to get under the seat (due to clearance) and satisfactorily and safely do the deck bolts. This also allowed me to slide shims under the front brackets to tilt the seat back just a few degrees. I had to do this because the rear hooks, when hooking into the original vehicle, were below the flush deck, and my bus deck doesn't have recesses latches like the van… so it would have tilted forward enough to be uncomfortable.
The seatbelt in the center has both ends… the belts to the r/l have one end each, and on either end of the bench is the attached (factory) corresponding end in a plastic sleeve. It is secure, and I have every reason to believe that it qualifies as street legal.
There were different vans and suburban seat options at the junk yard! But size, attached belts, and color all fell into place for me. Oh, and the back folds down still so I have an additional flat surface with a hard plastic platform for ??? use I need. I don't think anyone ever even sat in this seat… It's in perf condition.