If you are concerned about welding close to the trans, engine or, dash. You can get your replacement panels and glue them into place. I have seen a few body adhesive products that make attaching metal panels easier. I like to use the finished product I have thought about for months, as my motivation. Nothing like attacking one problem from a list at a time and then crossing each one out, until that list is real small.
I performed my rust repairs as I was gathering the parts to fix my car, The floors, door sills, battery box, and spare tire well. I cut the rust all out and sealed and riveted all the new sheet metal into place. It was never planned to be a show car, just a fun play toy that had the power to keep up with todays performance cars.
Attack those rust repairs.