Buses came with 350ci, 366ci, 427ci, and 454ci motors frequently. These were almost always the industrial version which was much stronger, but less powerful than the regular version. I think a 454 would move a bus along just fine, but don't expect mileage. The mileage was pretty bad in the lighter vehicles as it was. Of course there are about a million hop up parts out there for them. For instance...you could find a set of Vortec heads for that 454 off a newer big block to increase power AND mileage.
If I were you I'd be more worried about the tranny. A turbo 400 tranny just isn't enough for a fullsize bus. It would be more than adequate in a short bus. There is a turbo 475 tranny that was used in heavier rigs that is about as stout as you're going to get without getting into an Allison. It was essentially a TH-400, but it has better clutches and straight cut gears. Those things are NOISY! If the bus already has an Allison I'm sure the bellhousing for a 366 (pretty common) would work just fine for you.