Busses that size often don't require CDLs, definitely don't require a CDL B. A true bus is much, much bigger than a 350 (source, I drive a bus). An E350 has a GVWR of around 4-5 tons. I drive busses with GVWRs of 16-27 tons. (Their visibility is much, much better than this because the engine is in the back like basically all full busses)