Re: Placing a truck on the roof
well, i have seen more than my fair share of conversions done, with vw's especially. the most common method was right angle brackets attached to the roof beams. most people welded sections, almost like a long tack weld, to add strength. then they sealed it with caulk. i've also seen a couple welded all the way around the seam. another thing i've seen for extra sealing is the use of trailer roof sealant around the seam.
note: i don't personally vouch for the strength of the attachment methods used by others. i'm just describing what i've seen.
unfortunately i fell off the bus. and now i'm not even near the darn road! ;) ;)