As a trucker, I'm going to chime in here on the engine... There may be some parts that the 3406 and C-7 share, and the C-7 may even share some architecture with the 3406, but it is NOT a 3406. Caterpillar offerings in buses include the C-7 / 3116 / 3126, I don't believe you will find or ever hear of a factory 3406 in a skoolie. Much bigger, heavier, and capable engine.
"Cheese Wagon" <email@example.com>
Former owner - 1989 Ford B700 64-pass Blue Bird (Rest In Peace, Cheese Wagon)