Steel wool? I picked up some brass wool at HD a couple weeks back. Steel wool has so much surface area it seems to rust extremely easy.
My '76 has the box ends up by the hitch ball just like Don's. I had a lot of internal rust, whether from the seashore or parking long-term in overgrown grass. The hole for the breakaway wires was quite rude. I used the Por-15 Epoxy Putty
. Wear gloves!! (cheap in paint finishing departments or the livestock supply areas of our fleet stores) It was easy to work and I tried to get about 3/4" contact with the beam insides. Hardened in a couple hours and was superhard overnight. I did not seal the back end up under the belly wrap -- figured I didn't want to let moisture in the odd rivet hole and never let it back out.