I wouldn't want one of those harness bars, especially bolted to the rear arches. The forces in a crash can be massive.
Do it properly and tie the harness bar in with the roll cage.
I had a harness bar bolted to the arches in my old mini, I had a car crash into the rear quarter of it at around 30 mph, and it actually took the impact quite well, the bar didn't bend, the arches bent around it.