Was going to retain the adjustable runners for now, so your above-the-floor frame looks just the sort of thing that I need (as the bulge of the seat will still be above the level of the existing floor.
The tabs at the back where you have attached the frame are the ones I was talking about, although strangely mine are upside down with the opening of the channel facing up and sit flush to the floor
Looks like bridging the gap from chassis rail to chassis rail is the way forward though. Although after sitting in the car with no seat (nice and low and snug, the lowered floor does look tempting )
Cheers Kevin .