If you're going to weld anything into the car, put it on a frame machine first and make sure the car is straight. A car with bars welded in is almost impossible to get straight afterward. When I got taught to do cages, we took the wheels off and the car sat on a big level I beam jig.The doors were adjusted to close properly, then we welded the bars in. If the car is kinda warped and things don't close right, welding bars in kinda locks that into the car foreve, Even a rear 4 point adds a lot of rigidity.