I'll add something that may or may not be pertinent here......
"crush test" suggests how well a van modified or stock original will perform but what are we calling a "crush test" exactly. To my mind that term conjures an image where a load is applied directly on top of the van OR if it were flipped upside down and dropped from some height.
IF we're talking how well a modified or stock van remains intact during a roll-over the top would be the second element of the body to experience any force, the side wall(s) being the first. (For this example I'm speaking of the cargo area, between the B- and C-pillars; A->B- is assumed to remain intact per safety standards.) If an E-Series roof sheet metal is cut away it is slightly compromised because the sheet metal and horizontal ribs are gone. If I'm not mistaken a movable pop top has a bit of framing that is or should be attached to the upper body which would act ever so slightly to add some lost rigidity back into the body.
The biggest loss of structural rigidity occurs when/if the C-Pillar is modified (such as a fixed fiberglass raised roof) and that loss not rectified. I have two vans meeting that description, one a 2000 and a 2005. MOST converters who properly installed such roofs included the aforementioned "roll cage" welded onto the upper body AND a C-Pillar lintel that ties it all into the body. There is a loss of rigidity to the van body (between the B- and C-Pillars but not horribly significant. Another factor taken into consideration for that type modification was a FMVSS requiring the rear doors to have an upper latch as a stock van would have (driver's side).
The fiberglass roof cap is essentially useless in a roll-over crash but if the inner "roll cage" remains intact theoretically the upper structure would have enough remaining rigidity to somewhat keep the van body from completely collapsing on occupants. (Don't ask me how I know a van originally built WITH that "roll cage" installed is greatly compromised when it's removed---its too embarrassing to discuss. 'Nuff said.!
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This is long -winded andperhaps out of place here but IMHO its necessary to discuss the body areas we're most concerned with otherwise we're speaking of different body areas or parts.
HTH
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