I did this on my car, I simply cut a section of the seam away, an inch or two at a time, welded up that section and gradually worked all the way down the seam. I then used lead over the top, instead of filler/bondo as I suspect this would crack with the stress from the doors. Three years of use and no cracks so far so I think this method is fine.
That is the exact route I was planning on taking...
I cut about a 1/4 inch on each side of seam and welded a strip back in place and if done carefully can be metalfinished with no filler at all, My concept 67 has pictures of it finished on here
I saw the pictures on your other post, but do you have any pictures of the process? The way you're describing it (the way I'm seeing it), it seems like a lot of structural intergrity would be lost.