Depends on what it is you need done, or what you want done. If you're lucky you could find a junker jeep with a good side panel and floor to cut out. If you use a grinder with a cutting blade you can make short work of it. Then carefully match the pieces and weld in. But I also believe you can buy floor panels, and some body panels pre-fabed and you just cut what you need.
I had a second cousin that did body work, and he told me of a tool that sounded really handy that would put a lip in the steel. You would cut the replacement piece large, and the piece to replace a bit small, and then use this tool to put a lip on the edge of the body so the new piece could fit flush with the outer edge. Then tack weld it into place, you could even tack it on the back side if you preferred.