massive rust in the roof?

i just got a 1985 dodge d-150 pick up truck and the body is sound on this thing, but the roof by the what was left by the rain gutter down to the windshield (about 1-2"high by the with of the roof) is gone missing and no longer there(got the idea?) i ground down till i got to good metal but alot of this goes into the windshield rubber gasket. but right now i have no welder and the truck is not tagged so i cant take it to where a welder is. when. when the time comes i will get a new full roof out of a junk yard, but until that day comes and since i don't have a garage and the truck has to sit out side what can i do to minamize farther damage. i have thought about filling the void with either silcone or expaned a foam then covering that mess with either bondo or fiber glass or am i making the mess worst by doing that? the roof leaks like a sib but for now the for pans and cab corners are in very good shape.