Honestly, I wouldn't fit a 4 point to any road car. In order for them to be safe they have to be tightened so you can barely move. And, with a 4 point, when you snug up the shoulder straps the belt at your waist tends to creep up. If you really want a harness go for a 5 or 6 point, but again, they have to be very snug for them to work properly.
But, if you had your heart set on fitting them, I would think the safest mounting would be to a roll bar. The beaverboard firewall would be like mount the belt to cheese. You could add a plate to spread the load, but I still wouldn't trust it.