Here's and image you might find interesting.
The naked van scores 0.44Cd
The small pop top increases that to 0.45
The large pop top increases that to 0.51
The big high roof reduces CD to 0.40
and the big camper surprises with a 0.46
As for the airdam, I'd curve it if possible. All the new Euro vans have quite a curved front end in plan view.