I would check the doubler plate underneath which runs from one side to the other at the back, water gets in between there and causes crevice corrosion. I had to cut mine out reweld a section on then refit it as well two holes in my arches like your one. Here is a link for a the newzealand VOSA type peoples repair procedure, very handy. I managed without removing the body to chassis rear bolts but it was awkward. Heres hoping your ones ok.
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