There seems to be three different schools of thoughts on this topic.
First, is to do like the factory did and lead the seams and then do a skim coat of body filler.
Second, is to just fill the seams with bondo.
And third, is to weld in a filler panel and then bondo the seams.
After talking with my painter, who has been painting cars for over thirty years, I went with option III. So, I welded the seam closed with a small strip of metal and ground down the welds. I then cleaned the metal with metal prep,
http://www.halonmarketing.com/mall/Rust%20Away%20&%20Metal%20Prep.php (his recommendation) then I filled the area with metal to metal body filler,
http://www.parasolinc.com/Fillers.asp ,again his suggestion, and then skimmed it with skimmed it with Evercoat, liquid gold body filler. We spray it with a flat black this weekend. So far everything looks good no shadowing that I can see so far.