Damn RicerX hope to see you at the autocross this Saturday I want to see this setup.
But ya floor mounting can really mess you up. The reason is due to improper compression. Say you hit a wall while wearing your harness. The momentum of your body will be contained by the harness. If the harness is mounted to the floor your bodys forward force will cause the harness to compress your spine since its anchor point is far below you. Now if you mount the harness higher up say chest level you may still experience some compression but not to the extent you would with a floor mount.