To follow VEW's views, I have found that using the existing frame is much better. Use the van for axles and whatever, but keep the frame under the bus if still structually sound. Too many issues you haven't thought about when swapping frames. NOTHING fits. So measure and think about it. I don't know anything about the bus or the van. My axle/drivetrain swap experience is 55-59 light trucks.
I assume you have some big equipment and tools to do this heavy work.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn