I'm in total agreement with everyone concerning replacing the metal. I'm no body guy but I could create a filler piece for that in an hour wit a small hammer and anvil. It would take me a lot longer to weld it in, grind it down, seal it, fill it and make it good for paint.
If you need a temp fix, wire brush it down (high speed). Let the wire brush knock off everything it will! Chunks of metal etc.... (these are/were too weak to keep). Then use long haired body filler to "rebuild" the surface. It would be nice if you could get something behind the holes to support the filler. I'd try anything I could stuff behind the holes. Bear cans, Styrofoam, paper sacks or news paper. You're not going to leave that crap there; you just don't want to keep putting filler down into the holes. With the long hair filler, use a body file while it's still pliable so you won't have to do too much sanding.
Then, as soon as you can... cut that #$%$ out and replace the metal!