A single hoop will not really be much protection unless there are support bars that run forward or back from the main hoop. Be very careful as far as clearance to your head. If your car doesn't have seat belts, then you run the risk of smacking your "melon" against the bar when you hit a pothole, curb etc (assuming you are going to street drive it and won't be wearing a helmet). Cobra's have the roll bar hoop come up through the body behind the passenger compartment, that gets the bar far enough back so rollbar/melon contact is minimized. Something to think about. Do yourself a favour and DO NOT get a muffler shop to bend it. Most muffler shops use a bender that does not keep the tubing uniform in shape (what I mean is when a real roll bar tubing bender is used the tubing is the same shape and diameter before, during and after the bend). The result of a "muffler bend" is you end up with a crappy looking product.
Borntolose is VERY correct. In school when were studying chassis dynamic theory one of the things we put a lot of time into was why NOT to put a roll cage/bar into a street driven vehicle. A small accident can actually result in death specifically because of the roll cage. Roll bar padding does help, but does not eliminate the problem. Just make sure you get the bar away from your noodle, and that you cannot hit it, (your body will move a ton in the event of an accident) and wear a real harness, something that locks your shoulders to the seat. Other than that, have a diagonal brace and a horizontal tube at the same height as your shoulders to mount the harness to. Forward or rearward support tubes are a good idea, but not 100% necessary if the roll bar is done correctly. I'm putting a full cage in my model A, so as far as I'm concerned, if proper thought is put into it there is no reason you can't put a roll bar in a street car.
there's some in here along with links to more... http://www.jalopyjournal.com/forum/showthread.php?t=269648
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