I believe what you are speaking of is the "rocker panel"....its welded on as part of the truck cab. If its rusted you need to cut out all the rusted metal and sand down or grind any areas that havent rusted through. You have to be real careful with rust because its like a cancer that keeps coming back. The best way is to get a patch panel from another truck and weld it in place. If you dont have a welder you could use some pop rivets, but that would be a less than ideal repair. Most body shops charge 35-40 per hour I think and you could get them to weld a patch for you and then you can get some spray cans and paint it yourself. Then there is always bondo, fiberglass, or bondo-glass but that type of repair wouldnt last as long as with a metal patch.
It shouldn't be too bad. If you find a max or ram 50 in the junk yards with rockers in great shape. Unless it'll have to be a patch job. Gl.
Have the same problem as you. Clean out the body drains just under where it rusted, very likely why it did in the first place (full of water).
What I did to "fix" it was to cut out the rusted bit, sanded out the rust, and hit it with some rustoleum paint. Then rather than patching, I just got a couple pieces of stainless sheet metal cut and bent to fit, set it up for rivets, sealed with silicon and covered the hole. Looks like vanity trim and still works as a repair.
I have found that rust is one of my largest issues with my D50 as well. I have found cheap replacements for the front of the truck (quarter panels, bumper, doors even...), however I still have the bed of the truck that has significant work to be done. Would it be worth it to try to salvage and fix the spots, or do I just try to find a clean replacement at the salvage yard I can sand down and paint? I just don't have much experience with the pull apart places and I'm not quite sure if that would be a better route, or even cheaper in the long run.
I picked up a pair of aftermarket bedsides that sort of look like they would fit - with a lot of persuasion. My 1980 had 2 beds that rotted - one on the outside seams, the other on the floor seams. When my friend and I went and got the third bed, we were so concerned that the bed seams in and out were good, we forgot to check the inside of the gas filler door - after we got it to his garage, we opened the door, and it was all but rusted away - that is why my gas filler door is shaved and the neck is now on the inner fenderwell under my tonneau. We also shaved all the outside bed seams by welding them in and had a custom roll pan made and welded it in.
The best Dodge that Dodge never made
Living the D-50 lifestyle since 1980