[SteveB]

There is another aspect to this apart from the crash/ restraint issue.
If the seat is not integral a'la SPF or hard up against the bulkhead, the seat provides the support to hold the driver in a fixed location. When cornering forces, braking or clutch depression takes place, the effort is transmitted into the seat, tending to tilt the seat back and lift the front mountings... that are resisted by the floor mounts.
If there is space behind the seat it will move and tip back if the front mount fails..suddenly and could cause a momentry loss of control as the only way to stop falling back is to pull on the steering wheel.
Then you will rely the harness.

Steve