I made a 4 point harness utilizing the stock passenger seat belt.
I have the SCCA roll bar and I fabbed a set of bars which bolt from the top of the bar to the rear of the pan.
I took out the passenger seat belt and "striker" (word?).
I removed the lap belt and all that I was left with was a belt on a roller with the stock mount.
I drilled a hole in the harness hoop which sits behind the seat attached to the rollbar.
Then I drilled a hole in one of my fabbed pan bars.
The stock seatbelt mount bolts right to these.
For the striker, I cut into the rail by the door and bolted it through this hole.
So, in effect it is an extra belt which is not mandatory to use at all times.
Oh and the best part is, since they are both stock, they are fully retractable...