Having driven both the Transit EcoBoost and the 3.2 Puma diesel, I can say that either one of those is a suitable replacement to the current Sprinter class B's up to 22', and the smaller class C's.
The transmission/engine combo on both work way better together than the V6, NAG-1 trans combo does on the Sprinter.
The comparisons concerning the E-350/450 or Chevy 4500 chassis to the Transit are irrelevant to the Transit in it's current chassis weight capacity.
There is at least a 450 and maybe a 550 Transit option coming eventually....