I know I already said it, but I'll say it again,
You don't have that much rust there, just cut out a section and put in some 18 ga sheet. cut to fit and weld in. This will fix any strength problems, plus it doesn't cost anything to get a little sheet metal.
That is a very common area for rust because water does not drain for crap there.
I would advise drilling some uniform holes to help with drainage. that and some real good primer/sealer and paint as advised in several of the above posts.
good luck!