https://rivperformance.editboard.com/t7272-pcv-valve-location?highlight=pcv+valveIn this picture, the green arrow points to the plate you need to remove.
http://www.paulstravelpictures.com/GM-3800-II-PCV-Valve-Replacement-Guide/Pontiac-Grand-Prix-PCV-Valve-Replacement-003.jpg
http://www.paulstravelpictures.com/GM-3800-II-PCV-Valve-Replacement-Guide/Pontiac-Grand-Prix-PCV-Valve-Replacement-006.JPG
http://www.paulstravelpictures.com/GM-3800-II-PCV-Valve-Replacement-Guide/Pontiac-Grand-Prix-PCV-Valve-Replacement-007.JPG
http://www.paulstravelpictures.com/GM-3800-II-PCV-Valve-Replacement-Guide/Pontiac-Grand-Prix-PCV-Valve-Replacement-008.JPG
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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