See what your current power levels are. A lot of engines came in the 180 hp range. I think the dt360 came stock up to 210(pretty rare), going to 250 or so would make it a pretty good runner without sacrificing too much reliability.
You don't need 500hp and 1000+ ftlbs to make a bus a good runner.
Unless you're loaded to 40k lbs, then it's a different story.
Honestly, changing the gearing in a bus is a big step to drivability, most are gear bound to only go about 60 at rated rpm. Once a diesel engine gets near rated rpm, the power and torque fall off a cliff, which gives you the false impression that the bus is gutless. Adding more power to it won't change that, you'll still be gear bound. You have to put your cruise speed back into a decent rpm range. Do that and you'll be amazed at how much more driveable a bus is.