before id try finding an 8.3 and making it all work (time sand expense) id probably put that effort into turning up the 5.9.. you can slso get 6th gear unlocked on your 3060.. and if that cuases your highway RPM to be to low you can gear down the rear which would give you more torque to the tires..
you also have to figure that a school bus say 66 or 70 pasengers.. kids at 100 lbs a piece.. now minus the weight of the seats .. unless oyu are carrying hundreds of gallons of fuel or water or are using extremely heavy parts in your conversion you should be OK.. there are a lot of resources for turning up a 24 valve.. your allison should be able to handle any of the mild non extreme upgrades you would give to the motor.