Here's my advice from a rank amateur that just finished with a rust bucket GTO convertible. Leave as much original metal as possible and just cut up the replacement panels to just repair the bad areas.. Weld the patches in with double lap welds. Assuming there's enough solid metal left in the car, that will reduce issues with maintaining dimensions and distortion. Double lap welds are strong and won't be that noticeable if you make neat welds, grind them smooth and use some body filler.