The N/A Buick is a 'G'-body style. Some other G-Body styles are Pontiac Gran Prix, Oldsmobile Cutlass, Cheverelot Monte Carlo. Alot of the parts in these cars are interchangable.
The Olds 442 came with a duel exhaust system. So did the Monte Carlo SS. The Turbo version of the Buick also shared a duel exhaust system. With some light modifications, any off these systems can be made to fit onto an N/A.
The 'Y' pipe on these cars, (which came after the cat), splits off into a duel exhaust system. The trick is in the flange on the back of the cat. Some later N/A cats may just bolt on to a 'Y'-pipe.
The cats on Monte SS's and Buick turbo's won't work with an N/A because both ends of the cat have bolt on flanges. The earlier N/A's had a ball flange on the rear of the cat. I believe that the 442 'Y' pipe may work with this type of cat. All and all it's just a matter of finding the right cat.
Once you do the choice is in mufflers, (offset or inset). Tail-pipes, Monte Carlo SS or 442's. You can buy a cat-back system which is the same principle, (Hooker makes 'em for the Buicks). They are more expensive though.
I attached images of the systems with this post. Advance Auto still sells the 'Y' pipe for the 442 for $84.00 and the tail-pipes, ($27.00 each). :cheers: