The question becomes how fast do you want to go? An 8.3 Cummins should be able to pull 70+ comfortably ... *BUT*, it'll need the right gearing to do it. Most buses are geared down for lots of stop and go driving, one with an 8.3 might be geared for highway speeds.
Another thing to keep in mind. You're going to drink a lot more fuel maintaining higher speeds. The primary reason is drag. Sure, the mechanical parts have more drag at higher speeds, but not so much to cause that much more fuel consumption. Most of it is wind drag. Let's say you're cruising around at around 40 MPH. You have a modest amount of wind drag but for the most part the air flows around the bus reasonably well. Above this speed, you are not only pushing more air around the bus, but you are also doing it much more quickly. To some extent, it follows the square rule of physics. Double the speed, 4 times the work to maintain it. Triple the speed, 9 times the effort to maintain it. Also, keep in mind, not only are you pushing air out of the way, but you also have a "vacuum" following behind, the void behind the bus after it passes, but before the airflow can "close the gap", so to speak. Again, the faster you go, the larger it is, and the more effort that must be spent to maintain it.
This is a good explanation why many buses may get 9 ... 10 ... 11 ... more MPG cruising 55-60, but will drop a couple points just running 65. And they'll probably drop another point or so at 70. And drop to 5-6 MPG averaging 75. You get the idea.
I have a Suburban that's nearly 20 years old that gets like 20 MPG at highway speeds. Why do modern, high powered, highly tuned race cars drink 100+ gallons of high-octane fuel covering 500 miles? Even with their sleek, aerodynamic shapes, they still have a tremendous amount of wind drag to overcome.