05 f250 rear inner arch repair

So I'm working on my bed right now repairing The Rusted cross members only to find out that the inside of my Arch on the driver side is Rusted. So my question is after doing some research here and on Google, are most people replacing the inner fender with a new one or are they leaving it out to get the foam out of there so it will prevent it from rusting again and just welding a couple supports in? I know how the panel is replaced on the outside, im specifically talking about the inner fender.

Btw.. have all the tools to do everything including paint and I guess if I would have known in my bed was as bad as it is I would have got a new one but after buying all the material to replace the floor and the supports, I'm just too far into it now too stop.

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