[DougD]

I agree with MrFixit.
It is critically essential to mount the shoulder harnesses even with or up to 15 degrees higher than the shoulder height of the driver. This is why you see so many people use a bracket attached to the roll bar. I would NOT recommend mounting them through the rear bulkhead. If you mount them lower than shoulder height, physics will compress your spine during an accident. Not a trivial matter. The G forces involved multiply your body weight many times. The mounting point may do more damage that the accident itself would have. Get it right the first time. You never know when you'll need them.