It'd be a waste to have a 200r4 without the gears to back it up, and it'd be annoying to drive at more than 55mph with 3.42 (example) gears and them tiny tires. Plus, you can still use your stock crossmember (just cut a chunk out and weld in some flat stock like a g force cross member on the driver's side), or use a y pipe if you still have one, and run the exhaust like normal factory duals. Don't remember if you're using headers, if not, that's how I would (tbh wish I did) do it. You'd have to buy 2 different crossmembers the other way.
Afaik, every 350 buick y pipe was the same dimension, and it exits on the passenger side, so fitment shouldn't be an issue. Nor should finding one be as they were all the same.