Looks like someone put a quarter panel on it at one time. I would hit the roof seam with a rust converter before priming and filling. Dont go to crazy with the filler along the top of the quarter, its a hard area to sand.
my friend looked at it and we could only find the one seam it seemed like an odd place to cut to replace a panel. he says instead of body filler he could weld a strip of metal in the seam them a light coat of filler over that. would that be the eay to go?
