you can't run the shoulder belts on a 5 point harness without at least a 4 point cage. You may be able to run them all the way back to the package tray area and reinforce that, but it wouldn't be considered "cool". Mounting shoulder belts to the floor behind the driver is a bad idea. It compresses the driver in the event of a crash instead of holding them in place. I'd stick with the XV harness. On my street cars with cages I still run both sets of belts. Regular seat belts for the ease of mobility on the street and the harness for the track where you are locked in tight.