What I have done in the past (worked for me)
1. Create template to identify spot weld locations
2. Cut out rot - squared off for easier repair. Top and bottom if necessary. If your bottom piece is solid you are fortunate
3. Match sheet metal gauge, fabricate replacement pieces
4. Weld through primer on hidden areas
5. Weld new pieces in
6. Replace original spot welds. If you don't have or can't borrow a spot welder, these can be simulated. Search through old postings as this has been discussed a few times.
If there are alternative ideas out there someone will certainly chime in.