grind, weld and POR-15. Don't be afraid to cut the rust out, it has to go. You don't have to duplicate the old metal exactly though. Mig in strips where inside and outside panels are pointing AT each other, then butt weld PERPENDICULAR another strip the thickness of the 2 pieces where they are spotwelded together. Yeah, it's work, but it's probably a lot more doable than you think.

__________________
'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.