I have had to deal with rust in this location on two of my 1995 geo metros. Its not an easy task to fix.... and even after I fixed it, the frame still rusted apart several years later. Here are some pics if you're interested in seeing the process. This location on the frame is tricky because you have to cut pieces of metal to fit into curved surfaces and the metal is really thin so if your heat is too high you'll blast right through it. Make sure to give the metal time to cool between tacks.
You have to remove as much of the rust as possible while still leaving metal in key places to tack weld to. There is really no way to get all of the rust as shown on my car here.
The Driver's side
Then you cut small pieces of sheet metal to fit and just tack each one a few times so its in place.
The passenger's side had the same problem
After I tacked the metal in I filled in the cracks with metal putty. Looking back I think I should have just laid a bead instead.
Finally paint everything with a few coats of rustolium.
There was also rust under the radiator