I purchased the 24 inch Fiberine fixed top shell only on my 2006 EB.
I am 5'11" tall.
The center section is slightly higher giving plenty of headroom for a 6 ft. person.
But.... Once you start building, this changes fast.
My flooring is 5/8 inch thick
My headliner is approx 1/2" total thickness.
Insulation material takes another inch.
If you have shoes on you loose another 3/4" of clearance.
I can stand totally erect only in the center of the cabin. At either end I have to hunch down a little.
In my opinion, the 24 inch high top would be the minimum.
I decided on the Fiberine 24 inch top because on the more aerodynamic appeal it has. If I was full timing in the van, I would have done a top with more height and more vertical sides like the Transit top design.
Check my photo album here on the forum.