Allison’s MT643 transmission is a good choice for a skoolie. It’s still a 4 speed with the same ratios as the AT545, but with a much higher power capability and a lock-up function in 3rd and 4th gears. Combined with a rear axle ratio of 4.44:1 or lower, a bus equipped with an MT643 can have very good highway capability.
Linked below is the service manual for this unit to help you keep your transmission healthy.