you might have issues narrowing that box. Reason being, the cabinet depth on the srw bodies are shallower than the drw bodies. If you just take the width difference out of the floor, you may have suspension interference. (probably not an issue for you due to your fab skills and the fact you arent using conventional suspension).
The other thing that might help is when those bodies are made, they "hang" the side packs on the floor sub-assembly and then weld them in place. They arent solid welded iirc and you might be able to split the stitch welds and lift the cabinets off, mod the floor arrangement and hang them back on.