John, I respect that you want to meet the challenge of fabricating as much as possible and as "frugally" as you can. The area where that toe board, cowl, pillar, rocker, etc. etc. come together is critical to the overall alignment of the surrounding panels and structural integrity. I had the same scenario when I got into my '67 GTO. There are 7 LAYERS of metal all joined in that area. I took the "easy" way out and bought cut offs from donor cars. Makes the rest of the job much easier to work from.
The background somewhat hides details but the pillar piece on the left is what I cut out and the right is a mint donor from Oklahoma.
This is what I cut out to get to good metal. You can see what is left of the frame under the floor. It was completely rusted off.
With one, intact piece, I was able to move on to the body mount and outer rocker replacements. I staggered the cut to avoid one straight weld line inside and out. I felt this would provide more strength and minimize warpage from welding. I did this to both sides.
OK, no more "unsolicited advice " from me, unless you ask...