It's called a toe board or floor toe board. The originals were metal. It's not unusual to see wood replacements done by owners as a "field repair". Mine has a floorboard replaced with plywood and it works just fine. It's possible to find a NOS metal toe board. I think Obsolete Chevy still has them. It's hard for me to tell from some of your photos but you may need more than just the toe board.
