You really need to consider how you will use the car before you do this. They are hard to remove once installed and are really several steps toward a race only vehicle. Passengers in the back will be at risk of hitting the bar during an accident. The driver without a helmet or front passenger can hit their head backwards on the bar as well. There is a restrictiveness in the five point safety harness that goes along with the roll bar. Then climbing over the front bars for access for you and your passengers or having to deal with swing out bars is more fun. I only drove to the track or had an occasional joyride once the car was caged. Imagine squeezing yourself into the cage and harness at the local Lowes along with your packages. The grocery getting phase is definitely over at that point!
Even with retro-fits, our cars fall well short of the safety standards of today because of their design. Newer cars have crumple zones, appropriate seat belts and multiple airbags, and safety is a major focus. We were just not aware of these safety improvements when our cars were designed and they can not be effectively re-engineered into them.