Richard;
To restrict upper body movement I would suggest harnesses before the seat. Even though most seats do have some sort of shoulder "wings", to get some really good restraint out of a seat you will be spending some real money and the seat itself will be hard to get in and out of. The original 914 seat was redesigned because of ingress/egress issues.
Having said that, Corbeau Forza seats fit well in the car (many seats are too wide for the narrow channel the bucket neads to fit into without restricting the p-brake) and the price is good for us budget minded 914 drivers (
www.autotoystore.com delivered mine for just over $200.)
Then get harnesses and your driving will improve 200%. The steering wheel should be for turning the vehicle and you can't do that if you are using it to keep yourself in your seat. I learned that lesson the hard way after my first 2 day DE at Streets with stock seats and belts. My pecs hurt for 2 days afterward.
Good Luck