If you have the bed apart, consider one more step. On my 59 with a similar seam, I laid the bedside on sawhorses and after removing existing rust I drizzled Rustoleum red primer into the seam coat after coat until it quit seeping into the seam. Then sanded it smooth and followed up with epoxy and paint as described above. So far so good on the rust. I felt that if rust reoccurs it will come from the inside of the bed and it would be important to seal deep into the seam.
