Also, there are differnces in federal regulations for a new car from a factory to be "street legal" in order to be certified for sale, and local and state regulations for a car to pass inspections or meet equipment/emissions laws.
For example, a manufacturer has to meet certain crash standards for side intrusion, etc, but that doesn't mean that older cars or modded cars have to meet that rule. (i.e. you could in fact remove your airbags and side intrusion beams and still be completely street legal, but a car can't be BUILT that way now by a major manufacturer and be street legal)
In some states, you can take a new car, and put in a highly modified older engine and remove the catalytic converteers, and be street legal. in others, the engine has to be the same year of the car or newer with all the emissions controls it had when new in order to be street legal. In some states this car is completly street legal:
And in others it isn't.