In line with coilover and with your own assessment. One option would be to just cut a small part of the top off, a margin around the square hole. Then work on that piece and reweld it back on. Keep in mind you may not have to cut all the way out to the non bent area if you are able to reach in to work on the gentle/peripheral dents with a dolly and hammer or pry bar more easily after you cut it open. Cut this top off with a 1/16 thk wheel so as to create a small reweld gap. They do sell a .040 thick but it is shatter prone sometimes. It would be the best however. You can cut out two or three small areas if that is better.

Another option: It is also worth a try to fix the bends and the square hold flange without removing anything. 2 x 4's or combo of wood and metal levers can be used, with patience, to restore the shape almost completely due to the access thru the square hole.(unless access is closed off by the design of the pan). Some welding of any rips would be required. Just think on these options. These ideas are just reinforcing what you already know. You may develop much better solutions as you go than we can from our house.

Not sure if a plasma cutter would leave a small reweld gap. I have cut out dented areas, fixed them and used them as the patch, several times. When I do this I also place a inner flange to weld to. In your case, it seems you could reach in and use a copper backing when rewelding to eliminate blow outs.

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Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn