Welcome to the Stovebolt!
My understanding is that the 2-speed rear was an option on 2-ton trucks. My 6400 does not have one and I would not expect a 2-speed on a school bus.
If you have 100 50-lb bodies the cargo load is only 5,000 lbs. As the bodies get heavier the number you can fit on the bus shrinks. Plus bus drivers aren't as sophisticated as truck drivers and may not use the 2-speed at all, or may shift it incorrectly.
I don't know what bus buyers in the 50's actually did, but if it were my money I would not have spent it on 2-speed axles.
So this is all just my opinion, I hope its of some help!
