Honestly, you have no real idea what you have until you blast it. Once you get to where you need to replace panels, think hard about the structure as a whole. If you remove too much without supporting it, you can do a lot of damage. Doors won't fit after welding, etc. To prevent that, I usually weld an X brace in the door opening.
Especially before going into the internal structural panels. So, replace the outside panel first. Then you can see if the inner cowl structure needs replaced. Sandblast that area to see how far you have to go, then if the structure below the floor needs replaced, I would put in the X brace. Just simple warping that is common with MIG welding can move enough material to make your doors not fit. There is not a lot there that keeps the door opening aligned in the first place, so removing anything can cause problems. Take the door off first.
I have a project in the shop right now where the fellow before me X braced the door AND another one across the cab between the doors. It's probably overkill, but there is no such thing as underkill if you run into real problems after you are done. Take your time and it will come out great!
