ford 6.7 diesel into class c

The only diesel engines that Ford put in the E series (well, the fourth generation E-series, from 1997 on) were the 7.3L and 6.0L engines. Swapping one of them (say, from a cargo van) would be at least possible, particularly if it's from a close year. Some few class C's were made with them, even. I looked at a Gulfstream Ultra with the 7.3 that I would have been quite interested in if it didn't have very extensive water damage due to neglected maintenance.

As Trackrig says, the 6.7 won't work due to space constraints. (Ford gave that as the reason why it wouldn't be offered for the E-Series.)