Even a brace across the back would be unsafe. In no way should 4 or 5-point harnesses be used without rollover protection. They don't allow you enough freedom to move if the roof caves in on you in a rollover. The only thing allowed to move would be your neck and that doesn't move very far.
I found this guy on Pro-Touring.com. I will be going with this setup.
http://www.morrisclassicconcepts.com