Best way to fix my rusty bed?

[kingstrider]

Well I knew there was some rust in my bed around the tailgate but finally got around to pulling the plastic bedliner out to see what it was hiding. I'm guessing the bedliner was in there a long time which allowed water to collect around the tailgate and along the edges, otherwise the bed looks fairly solid and has what I think is the original paint. I'm thinking of either getting some patch panels from another bed or just replacing the whole floor if I can find a decent replacement. My biggest concern though is the flanges along the left inner wheel well and the right fender where they bolt to the bed. Does anyone have suggestions on how to fix these areas using the original sheet metal? I can replace the wheel wells if needed but want to keep the fenders. I'm thinking maybe I can weld in some L-channel and blend that in where needed but if there's a better way I'm all ears.