The car spent most of its life in Illinois, New Mexico, and Iowa. Im guessing minor rust started in Illinois and not much happened in New Mexico due to the dryer climate. Then it sat in storage in a damp Iowa building for a few months and it went down hill quick from there. It also caused mold to grow on literally every part of the interior. So I had to pull the interior for cleaning and have left it mostly out of the car since then for easier repairs.In all fairness, he probably had a clean chassis to begin with. I'm assuming that South Dakota winter roads are similar to Ontario winter roads?
Judging by your pictures you left your interior in place while you worked on your car? all of it? Some of it?