I guess if you had a look at a late bay that has a swivel passenger seat from the factory and replicated whatever bracing they use then youd be ok ... from what I recall its just a small triangular plate on the inner b post area.
If Im honest I dont think it would make 'that' much difference in reality ... would the van be affected in a crash? Maybe if you were hit on the side with sufficient impact, but I tend to think with such a force required then the difference in outcome would be negligable. Front or rear impacts? I cant see how it would really change things.
Would the van flex any more? Unlikely in most scenarios ... the axles/wheels are at a much lower point and are at chassis level, which if solid should prevent noticable flex. Im assuming its not a hi powered van being chucked about?
Would I do it? Not a chance .... you'll be cutting out massive chunks of value and desirability with every chunk of steel. Theres enough room in the back of a bay to seat 6(?) people (3 each way face to face) .... if thats not enough space then look for a Kempernink! :lol: