Well here is what I did:
I originally bought a 4 point autopower bar since I was autocrossing. I then took the car to a shop in the Manassas Va area where and engineer added the front hoop around the door and windshield frames and built in door bars. The reason the system was bolt in was because SCCA rules dictated it at the time. I used 1/4 inch backing plates and Grade 8 bolts. The car did not twist and the bolts did not work loose.
Now, questions have come up about welding in cages Vs. bolt in cages... Absolutely without a doubt, welding a cage into the car is much better than bolting it in. However, A bolt in cage is a better alternative than none at all. And Michale, as for its uses in day in, day out traffic, All I can say is that the state trooper, the insurance company, and everyone else who saw my car Knew that the cage saved me from personal injury. The rear bumper strut of the car was touching the strut tower, and the hood was hitting the windshield and roof. The motor was shoved into the firewall. All that damage occured when a 6500# truck hit me when he was doing in excess of 40mph and I was at a dead stop. He sandwhiched me between himself and a Ford Taurus Sedan. I walked away with a small bump on the forehead.
If you have a cage custom made for the car, and don't use door bars, you will have a very unobtrusive piece of safety equipment. I am a big advocate of their use, especially in the Z, and especially with anything over 200HP. Remember, the chassis wasn't designed for that kinda torque and HP that mild modified V8s put out. And if you paint a Z, spend a bunch of money on getting the body rust free and suspension and brake upgrades, I don't think you would want to twist it up and Use it like a plastic cup, tossing it away when its life is spent.
My $.02
------------------
"I will not be a spectator in the sport of life!"
mjk