Unless you're trying to win a season championship or something, don't worry about it. Just mod your car the way you want to and have fun. If your mods are obviously not stock, people will call you on it. If you change something subtle like springs and you're up front about it, most people still won't care as long as you're not taking away THEIR points towards a season championship.

To try to be clear, in stock class you can only run a car in a configuration that was available from the factory. If the parts in question were available as a stand-alone option (as an LSD might be on some cars), you could install it on a base model car. But if it's a set of springs that was only available by purchasing a car with a complete package, you have to run that complete package to be "legal".

Just go have some fun, would ya?