Converting a rig from gas to diesel sounds like way too much work to me. Why not spend a thousand bucks and buy a new bus with a diesel engine and air brakes already in it??
If fuel economy was the only factor, you'd be worlds ahead to use a diesel. The fuel economy in a bus can be almost double when switched from gas to diesel. There are downsides to a diesel.
I'd also like to recommend a DT series engine. (dt360, 366, 466) They're reliable, and CHEAP to work on. I'm finding out with the brazialian made ford that parts are very expensive compared to the international DT's.
Quis custodiet ipsos custodes (who will watch the watchmen?)