Originally Posted by BCam
You might want to PM JWA, a Sportsmobile member who used to own a body shop. He may have suggestions regarding the interior panels.
A bit late here but to start here's a link to my Imgur pages showing a few details of the raised roof as OP has---my 2003 E-350 EB is very typical of these types converted E-Series. https://imgur.com/a/iRmpb3n//?grid
The plastic interior trim IMHO is not only a space hog in general and hard to insulate if they're to be retained into whatever build out is going on.
Long story short the wall panels can be "easily" removed but anything like seat belt attachments and window latches need to be removed first. Attaching something to the raised roof's outer surface in a structural fashion would be extremely tricky I think because at this point I have no idea who close the steel "roll cage" and the top are to one another.
In my van I have a good 3-4" between the steel and the top inside, enough I can easily place standard R-19 faceless insulation bats with little trouble at the sides. On top of the "roll cage" the heights vary but it would range from an inch up to 4" or so--its sloped IOW.
So any questions OP feel free to PM me anytime--always glad to help if I can, at minimum share what I've done to my own E-350 EB.