The place to ask would be www.v8Buick.com. I'll ask over there and see if I can get an answer.
Here is the thread over on V8,
http://www.v8buick.com/showthread.php?248797-Power-Brake-Booster-for-63-Riviera&p=2034812#post2034812
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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