Over the years I have seen all sorts of power package "improvements".
Some were great and look like the factory set it up that way.
Others fall under the category of "what were they thinking?"
Most Crown and some Gillig schoolcoaches came from the factory with the engine mounted under the floor between the axles. In most cases the engines were rotated 75-90* and mounted pancake style. In order to mount them pancake style it required special parts and pieces not limited to motor mounts and oil pans. Even the bell housing to block mount is machined differently so that the transmissions are not clocked as well.
Way back the engines used were IHC RD450/501 I-6 and Hall Scott I-6 gas engines. The first diesel engines were Cummins 220 small can I-6 diesels. The factory even did a lot of repowers in the '60's and '70s to the small cam from the H-S gas engines. Sometime in the '60's the Detroit Diesel 6-71 became an option. When the small cam went out of production in the early '70's the Cummins big cam became an option. The buses built after the big cam became optional are easily identified by the flat top to the fenders. The big cam was enough wider that the frame had to ride higher in order to get enough ground clearance under the engine. By making the fenders flat on top it didn't require retooling to make new fenders.
If you have a donor engine with all of the pancake essentials you can put any big cam into a Crown or Gillig mid-engine bus. The kicker is there are very few applications that ever used the big cam in a pancake application.
I have also seen a Kenworth Pacific that had a big cam transplanted into it. It required extending the frame and adding a revised rear engine door. They basically built a dog house that stuck out from the back of the bus about 18". It also required fabricating a new oil pan because in order for it to work if they didn't the pan would drag on curb cuts. But I can tell you that old Pacific would go like sixty and never slowed down for anything.
The limiting factor for rear engine buses is how much room you have for a driveline. The factory driveline with the factory engine and transmission is usually pretty short.
The limiting factor for Type 'D' FE engines with the engine up front is how much of the entry you can use up with a longer engine. If you reloacte the door and don't use the factory service door for your entry you would have no problem. The driveline length is not an issue but how much weight you can put forward is a real issue. Moving the radiator forward would be problematic as you start to have weight balance issues that you don't have if you move a big engine further back on a rear engine bus. Since none of the engine and transmission are over the front axle moving more weight forward is going to make a bad situation worse.
As far as putting big engines into a Type 'C' the only real issue would be how big of a dog house you are willing to put up with. Most medium duty trucks use the same basic chassis as Type 'C' buses. Any engine offered in a medium duty will have factory motor and transmission mounts that will bolt into holes already in the frame rails.