I tried to make about 2 turns with my '32 Ford Roadster on the RTTC autocross.... the first right was fine - then the quick left-hander that followed just about tossed me out of the car. No seat belts and a flat bench seat! Not good! The only thing that saved me was the floor shift handle!
We had this same problem in the '62 Bubbletop build --- so Rodger (Ironworks) found some Mercedes Benz seats that had the shoulder harness built into the seat back. Those seats have a monstrous cast aluminum chassis. Of course, we'd have just used the frame and we'd have had the seats built for a little more support etc.