Re: the belting on GM's? Where was this attached to bus? Is it full width on the original install? Was it in contact with the bus or is a space left up there? And finally, how tall should it be, or how close to the ground when aired up?
Look up under the back end of your '04, just behind the wheel wells. You should see a piece of angle iron about 1"X1".
The flap hangs there. It is pretty much just a mud flap that runs the whole width of the bus.
On my 3610, it was about 4" off the ground and on my 4103 I'm going to make it even closer. Probably 2" or so. My theory being that the less air that can be pushed under the bus to the engine compartment, the more air that will be drawn through the radiator and moved over the engine for cooling.
I just looked in the master parts manual for the 4104 but didn't see it, probably because I'm not certain what GM called it.
I hope this helped,