Here are some pictures of what I did to fix a rust hole similar to yours. As you can see the rust went through three layers of metal- firewall, upper portion of the front frame rail, and the frame rail itself.
I also found out that the rust extended further down, sandwiched between the frame and the floors. Since I wanted to remove as much rust as possible, all of that had to go.
After the rusted metal was cut out, I fabricated a piece of metal to fit in the cut section. Wasn't difficult- just needed time to massage it in. This piece was later spot welded to the frame and seams welded. Will have to get around to griding it smooth.
I decided to ignore patching the metal at the top of the front frame rails. I did, however, use ospho and the eastwood internal frame coating in that area. I will probably spray some of this stuff inside the frame rails again once I drill access holes in the replacement frame rails.
Hope that helps.