Many, many moons ago I worked for a conversion company and installed "toppers" on vans. I've seen the same style toppers on Burbs, and can't imagine the process being all that different.
It was a fairly involved process to say the least. The overall structure of the vehicle relies heavily on the roof cross supports. And keep in mind, we were regulated by a safety standard in doing what we did. We removed all supports except the first one back from the front, and the last one forward of the rear. Leaving the front one left a "cab" over the driver and passenger seat, and leaving the rear one left the same configuration in back. I'm not sure if this was done from a safety standpoint, or getting the benefit of the "shelves" it left for accessories like tv's and such. The whole structure was then re-braced with a pre-engineered kit that bolted in. The sheet metal was removed in the space between the remaining supports, then the topper was set in place. It was secured using an aluminum channel and LOTS of screws that tapped into the body's remaining sheet-metal as well as into the structural supports in several places. Then a molding was snapped into the channel to hide the screw heads, leaving a functional but decorative trim piece around the base perimeter of the topper.
Everything done by the conversion industry is crash tested for safety before it is approved for general use. It was determined that in a roll-over or bad crash, the system described above would allow the topper, in worst case scenario, to rip the screws out of the fiberglass topper, and it would separate from the vehicle as one whole piece. But the overall structure of the van would maintain it's integrity. So as long as everyone in the van was buckled in properly there was no additional perceived danger to the passengers. To that, I said, "yeah right"!
I know, this really has squat to do with your project. Just thought I'd relate the overall process and see if there was anything you might glean from it.
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