If you're looking for something date coded correct to the car, then I agree with Kurt, go with a month prior. Something in the mid to late September (I) or early to mid October (J) range. But if you're looking for a transmission that would have been replaced under warranty, you can go up to 5 years after the car was built, but you'll need to find one with a CT code. The GM warranty period back then was 5/50 (5 years or 50k miles, whichever came first).
Ed