@jfrench Are those your original JDM seat brackets? They look like my CDN BJ70 brackets, which, I think are the same as Euro BJ73 brackets and they are mounted in between the wheel wells. Supposedly, the Euro BJ73 has more rear seat legroom but I have not confirmed.
I ask because my JDM BJ74 brackets are smaller and were positioned in front of the wheel wells. The brackets cannot just be just moved rearward.
Looking underneath the BJ74, there is just one added plate for reinforcement.
If the OP has the same, smaller brackets, I'd imagine he could just modify the brackets so they bolt in between the wheel wells, then move the seat 2-3" back and remount. It even looks like the undercarriage reinforcement plate would accommodate such.