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I did find some time on Christmas eve to go back and create and attach the flange on the back side of the firewall. I was looking at some other builders' photos and discovered that I had forgotten to do this. This process was very easy to do. I stuck the .75"x.75"x.0625" angle up to the back side of the firewall and marked the places on this angle where the firewall had flange cuts. Then, using a #40 drill bit, I drilled holes at these locations on the mounting side of the angle. I then used my band saw to cut slots up to these holes so I could bend the angle to match the contour of the firewall edge. Be careful not to take too much material out of the mounting side of the angle. You don't need to, as you do not bend the angle much to get it to take the shape of the firewall edge.