I’ve done a bunch of rust repairs on a Firebird and a GTO. My philosophy is to only remove the rot and piece in patches with lap joints. Leave as much original metal as possible. Chinese replacement panels are mostly crap so minimize them. I end up with solid sturdy repairs with less dimensional issues than replacing whole floors, trunk pans, and quarter panels. Doesn’t look as slick from the back or underside as the skilled professionals but good enough for me. For floor work, just make it sturdy. The top has carpet and people don’t slide underneath to look at the underside of the floor