Onto the left rear wheelwell: This relatively innocent looking hole is the proverbial tip of the iceberg. | 
Removing the doubler plate reveals multiple layers of damage directly underneath where the plates are due to go for the roll cage, and is by far the worst rust on the entire car. | 
Looking through the trunk, holes punched in the first piece will allow plug welding the next piece in place. |
Like the passenger side, a flanged part is tacked into place as well. Notice the part in the foreground, which will cover the hole, also has holes for plug welding to the underlying structure. | 
A look from the driver's side wheel well gives an idea how badly damaged this portion was. It's easy to get overwhelmed at this point, but remember to break the problem into small tasks and don't try to fix it all with one piece of sheetmetal. The first piece is already tacked back into place. |  |
Rebuilding a piece at a time and working within my skills, the structure is starting to take shape. Notice on the right side of the image how close the welds are to the undercoating--this makes it harder to weld as the undercoat burns off and pollutes the shielding gas. If possible leave at least an inch of clean material on either side of the weld. | 
Measured and evenly spaced holes punched with the Roper Whitney hand tool make the plug welds look much cleaner. | 
Butt-welding the patch panels has allowed me to very closely mimic the original geometry. It's tempting to keep grinding until the weld disappears, but likely you will grind through a good deal of sheet metal before the weld is completely gone, so resist the temptation and throw a skim coat of filler down before you paint if the repair must disappear.To the right of the image is the opening where the fuel tank filler neck pokes through. Notice I duplicated the flange around the opening--this is an easily overlooked detail, but it was fun to create and goes a long way to making the repair disappear back into the car. |