While it might be easier to start with a flat frame, it's not a big deal to make one flat.

The hardest part is going to be supporting the bus body while swapping. It's big and heavy.

Speaking of weight, how much does the bus weigh? What's the gvw? What's the gvw of the donor chassis? Is the tow rating high enough for what you want?

Personally I'd be looking at a newer class 6 chassis.