I could ramble on about this all day but look at your harness installation instructions and any SCCA or NHRA harness install requirements. The shoulder straps should be perpendicular to your spline. Too high and you will rattle around in the shoulder straps if they are ever needed. Too low and you'll suffer spinal compression. If you go with the bolt in harness bar check the height and I think you'll find it's a lot safer height if they are just inside the body. Take pictures and measurements from the side, door open, seated in usual position etc.
Here's a CCX with it in the trunk.
One more thing about these, I would not let anyone use shoulder straps to hold them up without a roll bar to protect them.