Guys, I wouldn't even spend five minutes welding up that hole or even thinking about it, I would replace the whole skin. Replacing the skin would be MUCH easier and you would end up with a much better end product.
If you remove both front and rear windows and roof moldings. Then clean the sealer out of the gutters and pinch weld areas in the windows. The hardest part is melting the lead out of the sail panel seams. Clean them down to bare metal. You will see the spot welds that are holding the roof on. These spot welds need to be drilled out. If you can, drill them from the bottom and don't go into the roof metal, just drill through the one layer of metal right in the middle of the weld. I like to look at this as "Unbolting" the panel, you simply drill out each weld like you were removing a bolt, it really is no difference at all. At each corner of the roof there is a MIG weld or it is Brazed I forget, but those areas need to be cut with a cut off wheel or in the tight spots you can use a burr cutter. Remember, you can grind the living crap out of the adjacent panel, you won't be using it anyway right? You could even cut with a cut off wheel along the spot welds and then after the roof is off the car separate that last bit. But it is best done while the roof is stable on the car and not sliding around on the floor of your garage. Then on the car you are putting it on, you will want to remove the roof by drilling out the spot welds from the top, drilling through the panel you are throwing away. Again, destroy the panel you are removing, grind any welds on THIS side and do what ever it takes to the panel you are throwing away so you don't damage the fresh metal below it. I use a long Snap On tools gasket scraper to slip in between the sheets of metal to separate them. After you have the panel removed you need to take a hammer and flatten out the areas that you have drilled out the welds. Then grind with a little 3" 50 grit disc any little pieces of metal that will interfere with the "new" roof laying flat on the mating surface. Then using a "Roloc" disc clean that mating surface GOOD on both the "new" roof and the car. Then brush or spray a light coat of weld through primer. Let it dry REAL good before you do any welding. And DO NOT apply it heavy, it will be a mutha to weld through.
Pay attention to any sealant or "foam" cushions that the factory put between the panels to support the roof skin. If they got damaged as you separated the panels, you will need to put something there like a thick urethane sealer or something to replicate the soft, NOT stiff support the factory had. BE CAREFUL that you don't have too much, or that you don't have two factory layers (from the "old" roof and the "new" roof) that could cause you LOADS of trouble with fit of the "new" roof.
Now, if you have done a nice clean job of removing the roof, there are only a few holes in it. You can do one of three things to reattach it. You can glue the seams at the gutter and at the window pinch welds and only weld the sail panels and at the corners of the roof. This is how I would do it if I were doing it at home and didn't have the spot welder I do at work. Or you could weld the sail panels and corners with the MIG and then bring the car to a shop with a spot welder to do the pinch welds. Or you could plug weld with the MIG all the pinch welds. I have to say this is HUGE overkill and gluing is the way to go.
If you go the gluing route, forget the part about weld through primer, you will only want it on the areas you WILL be welding. Depending on the brand of panel adheasive you use, you will want bare metal or epoxy primer where the surfaces will be bonded.
Don't ask about bonding the sail panels, you REALLY do need to weld those.
I am sure I left something out, but that is a start.