I see someone did the ole bolt-through-the-frame to get the seats stay in. You might be alright. Are you gonna do this with the engine and tranny in place. Be a lot easier if you pull it. But, it is not imperative that you do. Just makes for more room to work.
I strongly suspect that the perimeter of the floorboards are intact, meaning not a bunch of rust. Do you have a large capacity air compressor? You will need it if you are going to use a cutoff wheel. Those things, cheap as they are, eat a lot of air. If not, you can use a RotoZip with the angles cutoff attachment. Cheap tool. But eats discs like I eat candy. Buy lots. I found that a nibbler works great for cutting out the floorboard. You can buy an air nibbler that any small compressor can use. My nibbler fits on the end of my drill. Nibbler won't go over the few spots where the floorboard rests on frame, although you can lift it a bit with a pry bar. Nibbler sticks down about 1/4" below working surface. I think you can cut out the whole floorboard and leave a shelf of about 1" all around to weld to, more in the back where there is less rust.
I welded my first floorboard in this afternoon. What a feeling of accomplishment. When I get it further along, I will post pics and a little story, for color.