I just inherited a truck with the same motor. They really are phenomenal and that's coming from a GM guy. They aren't the most powerful out there, but I really do think they perform right up with with a 302 in every aspect except how often they require you to stop for fuel. It is not a bad choice though space to work on it can be a bit cramped given how long and tall the engine is. Of course, it's also much narrower and if you don't have to work on it it's a bit of a moot point.
it's an excelent motor, also known to us oldtimers as the 300/6. IIRC it makes as much as or more low end torque as a 460 v-8, and their are aftermarket parts available like headers and a 4bbl intake manifold.
good or bad on fuel is a relative question, but it should get 15 to 20 mpg if you don't abuse it, that's lousy milage compared to my TDI Golf @ 39 but reasonable for something that pshes that much wind
Its a great engine, probably one of the best gas engines ever built. I dont see how in the world you would get over 10 in any bus. Mine at 15k lbs, 5.9 Cummins @ 60 mph got 8.4mpg hand caluculated over 200 miles. My Powerstroke gets 13-14 and towing a really heavy load the other day @ 70mph I got 7mpg also hand calculated.
it was the gm I6 that had integral manifolds with egr that had cracking problems. the ford 460's with thinwall cast heads had trouble with the exhaust manifolds growing with heat and breaking the heads, antifreeze/coolant ignites at about 1750 F and the resulting fires were the big push to the diesel pickups.