i also went with the 24" bubble top from Fiberine for our 2010 Chevy AWD 1500 regular body van
. We had it installed by Van Specialties in Portland Oregon as we live on the coast. I am pleased with the quality of the fiberglass finish and design of the top. It's great to take a step back from the cab and stand up anytime you wish back there. According to the gang at VS keeping part of the original roof is a common practice. In our case with the RB van we have ~9 feet on the floor behind the seat backs. With our chosen design we have a stand up area ~6ft long and ~5ft wide which is plenty with our layout.
One thing, and i've got to get picks up on the upper storage areas in ours... We had them keep the roof area above the cab in tact and also from the rear doors to the first rib on the RB. These create two areas about 3 feet deep and ~20" high for storage use. In back we keep our two carry-on sized duffles and in from we have three plastic cabinets we keep personal items, towels, etc.
Here is a link to an image showing the rear shelf
before headliner/carpet was installed. the shelf is the white-old-exterior-roof area.