One of the good things about this experience is that you develop an appreciation for how things are done. For example, how you fabricate an replacement panel is a mystery, but once you know how it's done, it's hard to consider that you ever thought that there was another way of doing it.
For the near-side panel, for instance, Aaron and The Dude first had to make patterns out of wood that would accommodate the necessary flanges and bends that were required. For the gas filler hole they made the wood pattern and then screwed it on the inside. Then they bent the metal inwards. They removed the screws and the wood and welded the remaining screw holes closed. Of course! That's exactly how I would have done it!
The first photo below is a general before and after picture. It's satisfying to look at. The second shows the jig-forms that Aaron and The Dude made. The third shows these neat little things called clecos. They're used to keep sheet metal in place as it's being welded or riveted. They're used in aviation a lot, which is why my dad knew what they were. Aaron and The Dude use them to line up the part before welding it in place. The last photo is the result: a perfect weld line that isn't shrinked all up like a piece of high-carbon bacon.