Drivers Side Floor Pan Replacement

Are you just doing the front left section? If so, I’m not sure you’d need a ton of bracing. I’d trim up the new piece to the size you need it, then outline that onto the existing floor with a sharpie and cut a half inch inside the lines. Such that the new piece will then overlap the old floor by a half inch. Use a flange tool around the sides of the new piece so that it sets in the existing floor, then weld it up along the seem. The overlap will make it easier to weld. Just my opinion and how I would do it.