I had only found a 1" rear sway bar available to fit the Gbody car, but then I did find a 1 1/8" rear sway bar kit made by Suspension Techniques and purchased it from JEG'S. It's made for the 68-72 Chevelle. The only difference is the space between the mounting holes in the sway bar is 1/2" off. All you have to do is elongate the rear mounting holes in the sway bar and it will bolt on.
http://www.jegs.com/i/Suspension-Techniques/876/51050/10002/-1
Sortly after I purchased mine these became available on Ebay. It's a monster sized 1 3/8" rear bar that will bolt on.
http://www.google.com/url?q=http://cgi.ebay.com/ebaymotors/1-3%252F8%2522-G-Body-Rear-Sway-Bar-Buick-GN-Monte-Carlo-78-88_W0QQitemZ150366583206QQcmdZViewItemQQimsxZ20090818%3FIMSfp%3DTL0908181510001r30873&ei=Z52NSqzTII7ysgOs2PTcCQ&sa=X&oi=spellmeleon_result&resnum=2&ct=result&usg=AFQjCNGbs7le-twt7uZ2MI6nCHmQWDWtoQ