Most open track racers remove the stock seatbelts from the car and add a 1/4" thick steel plate in its place to fasten the 3" aircraft seatbelts to. They also use 5-pint harnesses as the extra strap is cheap & easy to install. A very large washer or short bar below the floor will anchor the short 5th (anti-submarine) strap. As for the shoulder harnesses, be very sure its well anchored, as forces on the system are unbelievable- unitl it happens to you. Some racing organizations don't let you run a 4- or 5-point systems unless there's a roll cage installed, on the premise that you are rigidly lashed in place with such harnesses, and without a roll cage, if you flip, the roof may come down enough to break your neck or spine. With the stock 3-point system, you have the option of 'heading for the basement' or leaning over the console if this happens
