This is what i do for a living...repair other people's Toyota 4x4's. The frame is box construction, half full of wet mud. It rusts from the inside-out, the outside looks great right up to the end, so if it looks that bad forget it. Here are some pics of my work...
The inside of the frame is 2-1/8" wide by 4-3/4" tall, so 2x4 box slides right in, with a little filling to do on the top. It usually doesn't rust out from the torsion-bar mounts forward, and that's a good thing, cause from there forward it gets complicated. Don't worry about the fancy curves, just cut out a wedge and weld to form the new frame. Feel free to ask questions, or have me do it for you for $800. But i'm located in Pekin, Illinois 61554, so that might not work.
