I asked this same question some time ago and yes, it got a little heated as one of the members was a bit adamant that the seam should be leaded which I do not want to do. Therefore, I have spent some time researching this subject and arrived at the following;
1. if you are merely wanting to fill the area without any additional welding, leading is the best choice (there is a lead free option from Eastwood). There is also All Metal, however it remains to be seen if this will not crack over time (some say it will, others say it wont). I attribute these responses to how well the roof and quarter panel are welded (either factory or elsewhere). Bottom line, it is clear that with at least some factory spot welds as well as some body shop welds, the roof panel can and does flex which will result in about anything other than lead to crack (it might make lead crack as well over time).
2. if you weld the entire seam, it "appear" that you are much freer to use pretty much whatever you want to fill the seam. That seam is a bit deep to just slather bondo in there and leave it, but that might be car dependent. If you are intent to fill this seam with some type of body filler, choose a good quality product. I would use something that builds and doesn't layer so it cracks and falls out.
3. If you weld the entire seam and/or fill the seam gap with sheet metal (essentially bridge the gap between the panels) you can use far less bondo and probably have less issue so long as your filler panels are just below the panel edges so there is some place for the filler to sit AND you do not pour too much heat into the panels and cause yourself even more issues. This is the path I am going because I believe that adding the panels will strengthen this area even more and make the body work easier and cleaner.
