Interesting direction you've taken. I hope I'm wrong, but I strongly suspect your flat roof panel is going to warp and oilcan during the welding and/or when you take it outside in the summertime and the sun heats it. The flanged joints are not going to be able to be stretched after welding. I think I would have looked for a panel truck roof or maybe a station wagon if you wanted a flatter profile so it would have had a compound crown to it. There's a good reason vehicles don't have perfectly flat panels.
As you've found and maverikman will find, the drip rails are not just an L or U shaped piece of metal welded to the body, but is a somewhat complex assembly of multiple panel edges spotwelded together inside the gutter (and then covered in sealer) that cannot be simply cut off.
Tom it looks like you are a experienced body man you make it look to easy.How did you form the roof was it just a flat sheet hand formed in place or did you use the fancy metal shaping tools.How did you mach up the two cabs from looking at mine the back is wider than the front?You should have your truck build put on one of the saturday truck shows keep up the good work.Keep on trucking