Ford could take a much larger chunk of Class 6 in the near future with their new F600 product. It is basically a F550 with heavier springs and tires allowing it into the Class 6 market space. Yes, the F650 will lose sale, but the F600 should capture all of those and more !
I am still surprised that the new gas V8 is only 7.3L. The F750 does cross into the bottom of the Class 8 market, but that engine is just not "big enough" for that market. The "bottom end" of that engine is over-built (deep skirt block, 6 bolt mains, forged crank). I keep wonder if, at least on paper, there isn't a longer stroke version that would push it over 8L !