It looks like the new Ford Transit chassis with the 3.5L EcoBoost® V6 option is coming up close to the specs of the V10, so power should be adequate with this engine option for most U.S. market Class C models.
However, as shown in this link even the very heaviest Transit chassis - the 350HD DRW - has a GVWR of only 10,360 lbs., which is too light for many Class C applications:
http://www.ford.com/commercial-trucks/transitcommercial/specifications/Perhaps the good old Ford E350 and E450 chassis are proving to be a tough chassis to replace merely to gain some improved fuel mileage.
