Just a few suggestions that you may have already thought of....
If you decide not to cut and patch the brace and floor section, be sure to coat them with a high quality epoxy primer, with a rust inhibitor, before you cover it with the patch. Something like POR 15 would also work.
On your welds, use a chunk of copper for a backer, where you can. It will absorb the heat and help prevent blow thru. I smashed a copper pipe flat, then used a couple of magnets to hold it in place.
I recommend leaving a little wiggle room, say 1/16", on your patch piece. That will allow room for it to expand, from the heat, without buckling. Also, unless you get really good, really fast, don't attempt to run beads longer than about 1/2" at a time, or you will blow thru. I found the best way was to jump around the patch with simple spot welds until they were about 1/2" apart. Then, fill in the gaps between the spots, again jumping from one side to the other, until it's all filled in.
I hope this helps. Learning to weld as I repaired my tub has been the most gratifying part of my restoration.