Well I do like people who think outside the box!
Let's cut the question into a few parts.
First: "larger Skoolie". How large?
So let's just say 30 odd foot, seven ton for giggles.
And now second, engine size.
Seem to recall some early GM highway coaches with wee 270 ci sixes. Although perhaps not the most stellar choice - some tandem class 4 dumps used 300 ci Ford inlines. Skoolies in the 70's (here) were typically gassers. 318 Chrysler, 345 Binders. Ford as I recall had something unfamiliar to me. Want to say something like 377 but probably way off and never seen one.
So will a 454 pull a bus? Oh hell yeah trivially even smogged to the teats. But you'll run out of gas on your way to the gas station.
Oh man, the TH-400 was something in it's day (still run one too in a 7300 pound P-30), and I'm tempted to say it would go for awhile .. Maybe even a long while. (sorry, if not much actual science to that particular declaration).
Slush boxes were typically factory designated for input shaft torque rather than what they could ultimately pull in the real world with some given axle ratio. The actual numbers, puffed up towing capacity aside .. tend to be somewhat vague.
That said .. Roughly and anecdotally .. the 400 will pull 4 tons gross all day long for 200,000 miles plus with 4.10's in a city delivery truck.
But my feeling is it's not near enough transmission for a full sized bus.
Nor would i even consider one. Six percent of your fuel just .. poof. And no overdrive.
Stranded in time.. Surrounded by evil.. Low on gas.