As far as insulation-I stripped my bus down to the ribs inside. Welded up a couple of cracks, then wire brushed, then Rustoleum spray painted. Then installed 1x3 fir strips horizontally every 16" to have a backing for the 1/8" plywood to attach to-used 1x4 ash for the overhead cabinets. Now have 2.25" gap that I had spray foam (American Spray Foam). I highly recommend using spray foam-gets into the gaps and tightens everything up, and no possibility of squeaking compared to hard foam block. This was in 1993, and still is good. Good Luck, TomC
