<<Since I have yet to see an undamaged body panel start rusting at its
center, all that is really needed is to coat the bottom of the panels on
the inside and, perhaps, spray a band of the rustproofing on the panel
seams.>>

This would be a great place to apply a coat of urethane bed liner.
(On both sides of the seam.)

It's tough, it's scuff resistant, good sound deadner, water proof, and
won't turn into a ball of ear wax and run when it gets hot.