Ben,

Make sure that at the least, you weld up the crack and fill the hole.

I don't like the idea of welding a plate over the area, as this introduces a spot for moisture to sit between the edges of the two surfaces, which then accelerates the rust process all over again, unless it is all treated properly from the beginning and all sealed up.

The better, and more long term idea, is to cut out the section, and weld-in a whole new section, treat it all, seal it up, and paint ( Utilizing some of the ideas already suggested )

Mic