I actually have the same question so I hope the OP won't mind me piggybacking on his question. I will say that my original preference in the past had been to butt weld in a quarter skin (full quarters are not available for my car). I've found it exceedingly difficulty to butt weld a seam and not have the seam pucker inward as the metal cools/shrinks. As previously mentioned you can get a nice result by hammer/dolly to stretch the shrinked metal back into shape. The problem I found is in quarter panel seams that extend forward of the wheel well. There's really no way to get behind the panel so therefore the seam can't be planished. I guess you just fill it in with filler.
Next time I do a skin, I'm gonna try lap welding. Not knowing much about, can anyone give us the basics. I assume that when you cut the original quarter out, you leave extra metal for the overlap. Flange the original metal with a pneumatic flanging tool. Is the skin held on just with a series of plug welds? How close should they be together? Then just finish with filler of choice?