I looked at the 545 specs and Allison calls for a max 445 ft.lb. of input torque. The mildest build of the DT466 is rated at 520 ft.lbs of torque.
I did a very brief search for skoolies with a DT466 and a 545 and didn't see any. The dozen or so that I looked at had MT643, MD3060,2500 or were simply listed as Auto.
I also came across the following discussion regarding transmissions and the DT466: Allison Transmission + Int'l Engine Question - School Bus Fleet Magazine Forums
"About the transmissions — I had a feeling that an AT-545 would be too small. I've noticed the AT-545s in the full-sized transit-style school buses around here don't seem to last very long. That's what concerned me about the used buses I drove yesterday. I don't want to buy something that's going to give me problems down the road (literally)."
Am I missing something??