if the roof is welded in sections its probably warped to buggery from the welding and the the fact it wasnt supported underneath when doin so, wood is there to stiffen it and stop it being oil canny, best bet would be to use daves suggestion and be done with it, looks like you got R's 2s support on it too. weld a whole new roof skin on in one piece from a wagon of some sort once youve got all the bows stiff, the skin wont warp if theyre stiff and resting on them when you weld and has no where to sink. hard to tell without pics