I recently removed the interior on my e350. The plastic side pieces are big and bulky, removing them will give you more space side to side to work with when building out your interior. I’m 5’10" and with my side panels out I can lay stretched out side to side. Also removing the original carpet and insulation will let you put in a subfloor (I used foamular insulation and 1/2" plywood) that you can screw into for whatever else you may want to build in there. This will limit the amount of holes you’ll have to put in your floor from screws.
