Using patch panels and retaining the original character is difficult. Wherever you weld, you will have to paint it to keep it from rusting and the new paint will clash with the old. You might just want to repair the floor. The roof is probably stronger than you think. It may just have surface rust. It was common for people to use the roof as a step when hauling hay or cattle. I would let someone who has body work experience look at the roof. It is quite possible that it can be bumped out.
