Hard to say. The easiest thing to do is to bolt the wheel on before mounting tires on them. Stock 1995-99 Buick wheels are 16 X 7" with +42mm offset, so back spacing is about 5.65". What do the Deville wheels measure?
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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