Hey, I know its not exhaust.. but I thought of these to consider for your Dash A/C and defrost...
the size looks workable for in the front area of a Bus too..
this is the inside unit... it has Floor / panel / Defrost outlets and a dash control.. you can duct it in however you like..
Evaporator 960 A - 50 001 703
seeing the underbody of your bus made me think of This condenser unit too because it mounts underneat the bus and doesnt require room in front of your engine radiator or having to try and locate in the skirting area...
Condensers 102 - 50 001 479
something to think about anyway.. im likely going to use the above condenser in mine