You might want to consider a Lazy Daze if you haven't already.
The headroom is there, except right under the air conditioner. Many models have either a long couch (that folds into a single bed) or 2 couches that fold together to make a huge (larger than king) bed. The downside is folding/unfolding the beds each day since the couch/bed is a dual use space.
Some models replace the overcab bed with a TV and cabinets, but still maintain the overcab profile. From what I've heard from other E-450 owners, everyone gets about the same fuel mileage regardless overcab or lack of.
www.lazydaze.com