I asked him how he did it back when I dropped my car off back when I moved from NY to Utah. He said they mount a rod or bar across the firewall. That's probably what I'll do, but I was looking for alternatives.
My seats are taller than this, coming right up to the bottom of the trim strip.
The shoulder straps on these belts are, like, 8' long. I was thinking about just anchoring them to the floor behind the seat. Or running a bar between those two frame members and anchoring them to that