Remember, that new veicles are designed to get crushed in an accident compared to the lead sleds of old. You can install racing seatbelts also. I have a 4 point set-up in my '54 from Crow Enterprises consisting of a Y shoulder harness (3 inch wide), a lap belt(3 inch wide) and an anti-sub belt(2 in. wide). I use the lap belt, the shoulder sits behind the seat, and haven't mounted the sub yet. After the headers and exhaust get updated I will probably be using the shoulder harness. I spent around $80.00 for it and they will customized to your specs.That's per side.I use that and the passenger uses the 3-point that the builder installed. Can't use a 3-point with corner windows. Check out my pics on the yahoo link.
