I wouldn't bother with the Dodge transmission. It's simply not heavy duty enough to last in a bus. For all the work you're going to be doing, it may be far easier and cheaper to simply find a 5.9 equipped air brake bus and modify that instead, or at least cannibalize it for all the parts you'll need (I could see this if you like the Chevy looks instead of something else).
Personally if I was going to do an engine swap, why bother with a 5.9? I mean, sure, parts are widely available but it's not a very powerful engine (not without some seriously life-shortening mods) and for about the same work I could drop in something like an ISX Cummins or similarly big motor. Good thing is, if I snag a wrecked truck to cannibalize all this from, I get the transmission, axle(s), fuel and air tanks too.