Originally Posted by ResIpsa

When you said your shoulder belts are attached to the OEM bar do you mean the silver body stiffening brace ( that is often incorrectly thought as a strut brace)? If that's what you mean then that is certainly not safe. Looking at the attachment points for that bar and the mounting screws Plus sheet metal brackets it doesn't look that strong.

I suspect that you would be safer reinstalling your three points for the trip to the cage builder. You will need to unbolt the seat belt buckle from your factory seat and fabricate a bracket to attach it to your planted base