Would be easier, faster and better to install a full panel than buttweld and mud up a patch.

Thanks for the replies. When my supplies arrive, they are going in.

No way am I taking the whole truck bed apart just to repair a little rust around the wheel arches.

You did not state how extensive the rust was. It's uncommon for those wheelwells to have holes big enough to throw a small dog through and the rest of the bed on it's way out too.