[thstone]

I had the same situation;

You can't use an over the shoulder racing harness with the stock seat because the shoulder harnesses will be pulled too far apart as they come over the outsides of the seat and might allow you to slide out of the harness in a incident. The shoulder harnesses need to come nearly straight over the tops of your shoulders to work properly - which is why almost all racing seats have cutouts or harness holes.

Also, a four point harness will not keep you from submarining out from under the harness.

Also, almost no organized groups (PCA, POC, NASA, SCCA, etc) will allow you to run a four-point harness.

So, the only real option to install a harness is to get a racing seat and a five-point (or 6-pt) harness. You can easily swap the seat out between events if desired.