The only advise I have is.............ONLY use as much of the "panel" as necessary. Leave old metal that is not rusted. But I would have to see the entire inside to elaborate and confirm. I have patch repaired a lot of TF trucks, but not that piece.
Patch panel repair is pretty much all the same. All off shore parts. Need tweaking sometimes. Modified. Remove a little of the old at a time, bla bla bla.
Careful planning, measuring, template making, cutting, welding, are all the same. If you can't get behind it with a backer, you better know how to weld thin sheet metal.