Like EMRider said, the 6.7L is too large for the E-series chassis.
It's unclear to me how many customers would need 700+ ft-lbs
of torque in a van, anyway. If you do for some reason, you could
buy an F450 cab/chassis and add a cube van body to it.
I think Ford's original plan was to offer a 4.4L turbodiesel with
about 500 ft-lbs of torque in the E-series. Ford of Europe makes
a 3.6L V8 turbodiesel. This model was expected to be the basis
for the 4.4L. Ford even showed pre-production versions of the
4.4L in 2008.
Then diesel fuel prices skyrocketed in 2008, and Ford put the 4.4L
engine project on the shelf. Today, they're looking at the EcoBoost
gas engine technology. The F150 will get a 3.5L EcoBoost V6 in 2011,
producing 355 hp and 350 lb-ft of torque. This level of torque is available
from 1500 to 5250 rpm. These numbers are as good as those produced
by the 5.4L V8 currently offered in the E-series vans for 2010.
I expect that Ford will eventually offer the EcoBoost technology on
a V8 engine. In this form, the V8 will produce close to 500 hp and
500 ft-lbs, which would make the 4.4L V8 turbodiesel and today's V10
gas engine superfluous.