Yes, the limited-slip center section was an option, and they're not too hard to find. There's a difference in the ring gear spacing depending on gear ratio, but I'm not sure where the break point is. Sevral aftermarket suppliers offer posi units, but they're pretty pricey. Try the JEGS catalog. They've got a couple of different brands of posi units, or a check with your local Chevy dealer's parts department will probably get you some info on the factory unit.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!