You'll need a 98 FSM. Here's a killer deal. I'd jump on this,
https://www.ebay.com/itm/1998-Oldsmobile-Aurora-Buick-Riviera-G-Platform-GM-Factory-Service-Manual/114189678012?fits=Year%3A1998%7CModel%3ARiviera%7CMake%3ABuick&hash=item1a963c35bc:g:J8sAAOSwg6VemICT
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98 Riviera SC3800 All stock except gutted air box.
1970 Buick GS455 Stage1, TSP built 470BBB, 602HP/589TQ
Best MPH, 116.06 MPH, Best ET, 11.54
https://www.youtube.com/watch?v=UHCda-t_Jls
https://www.youtube.com/watch?v=sfT2tEO4XcU