All you need to do is measure the metal tubular spacer in the middle between the two metal washers.
Position the wheel in a way that you can do it more easily.
I believe the closest you will find will be these, 3 9/16" 9.8124.
It's not an exacting measurement 1/16" to 1/8" error won't kill you especially when you see what sizes they come in, one is the obvious choice to use.
I can recheck mine tomorrow, but it's a 1987 car.
Hot airs may have a different one then again they may not.
Energy suspension should have the exact number/size on their Buick info. page since they sell that master group of parts that includes end links for the GN's.