Back in the day, when I was looking, that part of the floor was the only section I could not find. It seemed nearly every other part of the body had been duplicated and offered. What seemed to me to be the only solution was to order a complete floor. I used the aftermarket M38 floors as that was all I could find close to the 3B. Besides, I had rot inside the tool box and under the tank, so a full floor made sense.
I did replace the thin, cheesy hat channels with Classic Enterprise versions. Also, I marked where I wanted to splice so I could keep as much of the original as possible, i.e. toe boards, transmission opening etc.
Some images to show what I did:
I personally thought the edges were a bit sharper than 3B floors so got the body shop to round them a bit next to the tank (the edges inside the toolbox wouldn't be seen). The only thing left to do in the bottom image was to canibalize the center hat channel section (also at the time unavailable).
One thing I will mention is if you're replacing the section under the tank, that body mount doesn't go through. There's a capture nut inside the hat channel so nothing sticks up under the tank.
