I wouldn't do anything to it, Just give it to me
You are better off fabricating new pieces from 1.1 Zincanneal sheet, and weld them in. If you do what Dan suggests (and don't get me wrong in what he is saying, his practice has applications) In the area your targeting, is quite rusty, not to bad on the outside, but eaten pretty bad on the inside, where you cant see and if you plug weld so to speak, you will fill with steel (no questions asked) but the panel will rust before your very eyes, where you have repaired it.
There is a certain amount of steel that you can weld into rusty panel without having the original sections of the panel rusting further, this is aided in the increased temp of the steel when its welded and the natural oils in the steel sweat out, leaving them unprotected.
Its the same when you heat steel sheet, and leave it unprotected, it rust very quickly compared to a piece that hasn't been heated.
And in the area your repairing is pretty hard to protect it from the inside. Thats just my opinion.
Regards John