I don't think the Transit is intended to replace the E450 V10 or for the matter the E350 anytime soon. I think it's intended to go up against the Sprinter chassis. From Fords press release the Transit chassis will only handle gross vehicle weight ratings from 9,000 pounds to 10,360 pounds.
"The chassis cab and cutaway will be offered in three wheelbases, 138, 156 or 178 inches, and gross vehicle weight ratings from 9,000 pounds to 10,360 pounds."
The 2014 Pleasure way Pursuit has a GVWR of 12,500 Lbs. It's cargo capacity is 2,100 Lbs. which means it weighs around 10,400 Lbs. So it looks like it's already over the rating of the Transit chassis (per Ford) by 40 Lbs. when empty.