Starting on the right rear wheel house, I removed the doubler plate that connects the trunk floor to the inner wheel house (see right side of lead photo), tar and seam sealer is removed before assessing how and where to start cutting. Angle grinder-mounted wire wheels, putty knives, and screwdrivers can be used to pry the stuff out, and a heat gun can be used to soften it up. | 
Before the doubler plate can be removed, the spot welds that hold it in place are going to be drilled out. There are several spot weld drills on the market, but I used a sharp drill bit of slightly larger diameter than the spot weld. Notice the dark circle in the middle of the cut; this tells me I have drilled through the weld. | 
A large screwdriver, stout putty knife, or hammer and chisel might be needed to separate the panels after drilling out the spot welds. |
Prying away the doubler plate reveals multiple layers of rusted steel that were only visible as a small hole from the inside of the wheel well; this side will turn out to be in much better shape than the driver side. | 
Permanent marker is used to mark the cut, which is made using the angle grinder with a .120-inch thick cutoff wheel. | 
After removing the rusted metal, I try to duplicate the structure of the material that was removed, which can be a little like putting a puzzle back together. I used a basic hammer forming technique and sandwiched a piece of 18 gauge between some four inch square structural tubing and a bit of flat stock. Notice the nice radius on the structural steel; I will duplicate this in my sheet metal by working the metal over the corner with the chunk of aluminum, or "corking tool". The flat stock on top of the sandwich holds the work as I hammer it over the edge. The end result is all that matters--be creative with the tools or shapes you have laying around to get there. |
Mocking up the rough part, it's close, but needs a bit of a radius to match the contour of the existing structure--a few minutes in the shrinker should fix this. | 
Looking through the wheel well you can see the teeth marks left in the work by the shrinker and how it draws the material together to create the desired radius. | 
Before cutting a part from steel, its best to mock up a prototype. In this case I used clear plastic sourced from www.mcmcastercarr.com to create a template and transfer the shape to the sheetmetal part. |