I believe that around 250hp and 660lb/ft torque is realistically the highest you can go with the 5.9 liter.
How much do the coach engines differ from the pickup truck engines? I'm guessing not that much, and I think those numbers are low as a result.
With H&S tuning, my Cummins ISB 6.7 (Dodge Ram pickup) dynos at 500hp and 1100lb-ft, at the rear wheels. You do the math on what that means at the flywheel. Heck, the engine is now available at 385hp/850lb-ft straight from the manufacturer. The 6.7 is mostly just a bigger 5.9, which a lot of government-mandated crap attached. I've got 57k miles and counting on mine, including a good bit of heavy towing.
Gearing and transmission durability are another question...