To add to what Schurkey said, factory GM service manuals can be found here in PDF format for free.
https://www.gmt400.com/threads/88-95-service-manuals.43575/
https://www.gmt400.com/threads/88-95-service-manuals.43575/
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As I said, it's been ~20 years...but I don't remember that at all.
Might it be that a 2 1/4" pipe would be flared-out to take a 2 1/2" donut