FWIW my shorty IH weighs 14180 pounds, 25,500 GVWR. I have the T444e and 5-speed Spicer with air brakes.
Ultimately to make OP's bus better suited for their wishes a repower is in order but while we have the 8.2 out, why bother with insufficient gas engines? And let's face it, no GM 1-ton truck transmission is going to last much time in a 17,000 pound+ school bus. So why not move to something *MUCH* beefier like an M11 Cummins cranked up to 400 HP? If that's not your thing, a 60-series Detroit nearing 500 HP? Maybe an ISX is your preference at 575 HP? Maybe you'll prefer a Cat engine? Scavenge one from a wrecked 18-wheeler and get the associated trans while you're at it.