I think that the steel on my 78 fj 40 was 18 gauge.
This steel can be bent quite easily around a form, such as a pole, while it is a flat sheet. Once the bottom 90 degree bends are there for the wheel well opening, etc., it will be pretty tough to bend. I don't recomend bending it while welding. Bend it first as close as you can get it, so that the shape is set, then weld it in.
I made my rear quaters by cutting out most of the old panel, including all of the rust. I bought a sheet of 4' by 8' 18 gauge steel and cut out the rough shape of the large hole (almost all of the panel. Then I made a template of the wheelwell opening with pieces of 1/4 inch thick by 4" wide strip, and clamped that to inside of what would become the panel, using pieces of the strip on top also to make a sandwitch. I then took a body hammer and gradually hammered the lip down to form the 90 degree bend. On my cruiser, I salvaged the bend at the bottem of the curved region of the panel and welded it on. I cut out the final shape by clamping the patch panel in place and cutting it to exact size by simutaneously cutting the end strips of the new panel and what was left on the car of the panel simutaneously with a jig saw.
What you need to remember, is that the fj-40 is more like a model T than a modern car, you can actually fix the body using some pretty basic tools.
Now for welding. Tig would be preferable, but I expect that mig is much more likely. The problem with Mig, is that once you push the button, the wire starts comming out, and material start building up. So to but weld on a panel you want close fit. Before you begin to weld you need to hold in the panel as best as you can. The problem is that no one person has enough clamps. What i did was to drill holes near the seam that I could insert a small screw with a nut on the other end and two big washers the material is held together by the washers. One of these every 4" is about right. Then, with the Mig, apply a small tach weld every 4". Make sure that the panel dosn't get hot.
The last thing you want to do is weld fast so that the hole panel gets hot and expands, then is welded in place. This locks in the expansion, so that when the metal cools it is under stress.
Next add a tack between the tacks. Now there is a tack every 2". Take a grinder and grind off all the extra metal. Next, take a hammer and dolly and gently flatten out the welds, so that the metal is in exactly the same plane on both sides of the weld. Then a tack between each of the welds. grind and hammer again for a tack every one inch. If you have access to a tig machine, you could weld between all the spots. With a mig, it would take some real skill to but weld in a sheet for the whole length of the weld without a lot of distortion.
Hope this helps, and have fun.
