Re: Advice on raising the roof?
I am guessing the available metal pieces are the side rails from standard 19" or 23" open "relay racks" used to house electronic equipment manufactured in 1-3/4 inch increments. How tall are the racks? If they are 7-foot or 8-foot racks, and you can access a lot of them, how about using an entire side rail at each hat-channel post running from floor to ceiling on the inside of the bus, instead of cutting the rails up for outside spacers. This plan assumes you are pulling out the inside steel wall panels to add Alaska-grade insulation in the walls, and putting up new interior wall material over the insulation. The flat surfaces of the rails would provide great attachment points for wall materials. They would also support attaching wooden studs to minimize thermal bridging and increase insulation space. The continuous rails would add stability to the bus structure, as well.
Just thinking out loud here.
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.