uhhhh... actually, you really should never put a skin directly overtop of the old panel as you create a pocket for dirt, moisture adn rust to form, this only makes the new panel rot out twice as fast as the old one. to properly reskin a 1/4, you cut out the "skin" of the old quarter, which is just the outside part, and trim your new skin to fit directly edge to edge with the old remaining section. this gives youa very nice fit and finsh, with absolutely no place for rust to hide. There are other types of welds you can use to attach a skin. there is the flange welding method, plug welding, splicing, resistance spot welding, and gluing. if the car will not be a show car, but will be driven occasionally to cruises and out to bang the old lady in the park after dark, then I would go with panel bonding glue, like fusor, or 3m, or SEM, or Trans-star panel bonding adhesive. you have a lot more flexibility, and honestly, this is the best method to use when reskinning, for durability. infact, for warranty purposes, factories encourage the use of panel bonding adhesive, opposed to welding for collision repair. you might want to look into this method if your car will not be judged, or heck, even if it is judged, I would still do it, because you can hide the flange in the runk, with good body filler and some fancy sanding. I am thinking of doing this to all my own stangs now, after seeing a demonstration given at a tradeshow.
the demonstration consisted of a twin tower 50 ton frame machine, and 2 panels, glued together with trans-star panel bonding adhesive. they attached Mo-clamps to both panels and chained the clamps to the towers and "gave it a little tug"
in teh end, the air over hydraulic 50 ton rams managed to tear the steel of the panels, and the glued seam was not even streached.
can't do much more convincing than that!!
also, panel bonding adhesive is water tight, so, no rust...
just another random thought from a random bodyman.