When you build your stock car, the first and arguably most important project will be your roll bar/cage. The roll bar/cage is meant to protect the driver, especially during a crash. The cage also has the added use of making the chassis sturdier, so it won't bend easily. Over time both will be important.
Your first decision will be on whether to build a roll bar or a roll cage. A roll bar is a basic frame with 4 points. Points refer to the points at which it will be welded to the floor. A roll cage has more points (6 points and above) and bars protecting the driver. Different races have different rules on the number of points a stock car should be fitted with, so make sure you consult the rule book before you get started. Beyond what the rule book stipulates, the kind of roll bar/cage you build is completely up to you.
The material that you can use to build a roll bar/cage is also restricted by the rule book. Most of the time you will have to choose between mild steel or chromoly. The choice of which material to use boils down to a tradeoff between cost and speed. Mild steel is the cheaper of the two but it also weighs more. So a racer on a tight budget will likely choose mild steel being mindful that it will cost him/her speed. Chromoly has more strength than mild steel, so less material is needed to build the cage. None the less most drivers opt for mild steel.
Having more points on your roll bar/cage not only increases the safety of the driver, it also adds structural integrity to the chassis. This becomes more important with cars that have a lot of horsepower and torque which naturally tries to twist the chassis.
There are cages that are sold as first or second hand. If you choose to buy a kit, ensure that it will fit your vehicle and be up to code with regulations. Alternatively, you could hire a welder to do the job for you, but this isnâ€™t a job for a first timer. Racing magazines and websites are filled with craftsmen looking for work, but your best bet is inquiring at the race tracks.