For the side walls, I ripped 2 x 4 down to 1 inch, making furring strips that were 1" x 1-1/2" x 18".(1" was the depth of the seat support rail running the length of the bus.) Pre-drilled then attached with "wood to metal" self drilling screws into the ribs which are in between each window. There were a couple places where my wall covering material ended midway under a window, that metal is thin, I used the same furring strips but attached them with regular sheet metal screws which have much coarser threads.
For interior wall framing, I used 2x4 that I attached to the floor with a cleat,(made some of my own, bought some from Lowes) The top of these 2x4's are attached to the roof rib with a carriage bolt. I did have 1 place where the top of the 2x4 didn't meet a rib. For that situation, you can span the distance between 2 ribs with a 2x4 then attach to that. I posted pictures in my build thread.
Liability disclaimer; I am NOT a structural engineer, my bus hasn't left my driveway since my I bought it so I don't know how all this will work out 10,000 miles from now.
Use the search window on this site, type in "attaching walls", you'll get a bunch of ideas from a lot of people who have already traveled this road.