i'm going to fit a spreader plate in behind my fuel tank, but only as i have it out. i'd rather have peace of mind that there's something fairly strong there to hold the mounts in with!
I had a roll cage fitted into my mk1 golf and the company fitted the 6 point harnesses for me. They used similar mount points to these.
I have to say I have seam welded my seat belt mounts straight into the bulkhead at the back and onto the wheel arches! Another point that was made at the time I was doing mine was it is not just about the seatbelt mount safety but also the mount of the rock and roll bed cos you will be I big trouble if the seat belts stay put but the bench seat doesn't!!
if a 1" plate is welded to 1mm steel then its only as strong as the 1mm steel and the 4" of weld around it. if a larger plate is used the stress is spread over a larger area so is stronger!