Jeff Cornish is big into the '68-9 GS Buicks you can go to his web page at:
http://www.flash.net/~stanbrad/cornish2.htm
Yes, these were exclusive to the GS models and were used (in a couple different variations) from 1969-1972. To see what one looks like on my '71 GS455, go to my web page (under sig file).
That could be due to the fact that your GS may be a 400 (unless it's a California 350), and the Skylark is a 350. These are different motors! They can be swapped rather easily, but you're ruining the value of your GS if you're going from a 400 to a 350, without planning on keeping the original (assuming it is) engine in it.
They use different frame mounts, and the TH400 trans that comes attached to the GS400 has a different kickdown mechanism than the TH350 trans used in 350 cars.
Also, the exhaust won't hook up.
You're best off finding another 400 or 455 for the car instead of going down to a 350.
For more Buick info, go to
http://www.buickperformance.com
Steve '71 Buick GS 455 Had: '86 & '88 5.0 coupes (ex-staters) <a target="_blank" rel="nofollow" href='http://members.aol.com/SEEverist/index.html'>
Steve's GS Page</a>