As I understand the rules, they state that you can fit your car with whatever options were available from the manufacturer at that time. That means I could retrofit my base model 240 with a sunroof, "sport suspension", lsd, abs, and hicas, and still compete in G Stock.
The one catch is that you must install the whole package and not just parts. For your LSD, you'd also have to install HICAS because in '92 (my car), an LSD was only available in the handling package which included HICAS.
You could install an LSD and race in STX though regardless of what your car originally was.
check
www.socal240sx.org for details regarding your available option packages.