The new Transit van has the tallest interior, so that's what I've focused on.
Maybe you already know this, but the Sprinter has a tall roof version with a 87.3" (7'3") interior, which is almost 6" taller then the biggest Transit. Just incase you want to stand up and swing your arms around a bit...
The Sprinter super high roof is also non-structural fiberglass. So cabinets and upper mountings cannot be anchored directly as they would in other vans.
Advanced RV did convert a super high top but they had to build a structural subframe underneath the van top. Interesting concept, but starting price tag was $250k+