You will want to isolate the heat as much as possible, copper backing will help with that a bunch. You may have to shape the copper to fit the contours as needed. If you can find them, 6013 1/16" will do it. Obviously not ideal, and you WILL have to use some filler (bondo) after grinding. MIG is a better option because you can control the heat to a much more confined area. TIG in something like this is a pain, you need a helper to hold the copper backing while you use both hands to heat/fill the holes. At one point I had a '74 Maverick which was an LDO (luxury decor option), which meant it had a vinyl top, thicker glass, and a piece of trim molding all the way down the side, doors, fenders, and quarters. That trim was screwed on from the back side, so there were about 30 holes in each side to fill. All I had to deal with was an old Craftsman (I mean really old) stick welder, the kind that you had to remove the lead from one hole an place it into another to change amperage. I hated that thing but it's all I had. With a piece of copper pipe beat down flat over some railroad iron, I used it for backing and 1/16" 6013 to fill every single hole, then grinding, then filling each hole. On the back side, you'll also want to get some corrosion protection on it to keep it from corroding. It wasn't that hard, and came out fine. You DO have to be mindful of the heat because obviously if you camp out too long you're going to either (1) burn through or (2) warp the sheet metal. Or in my case (3) all of the above. The good thing is that material can be added back in, although that's not the ideal solution, it works. Mine came out fine, could not see any of the filled holes (at all) after the color and clear were laid down. Nobody but me knew it was an original LDO.
