the best thing is to put on a rear sway bar and some polyurathane bushings. From what they say it will reduce avout 75-85% of the rearend hopping that you are getting. Common problem with IRS but fairly easely fixed, SLP and Hotchkis both make kits fot the GTO but it's realy up to the indevidual which one you go with. I dont know much about SLP but havnt heard anything bad. I had a 71 Cutlass and the Hotchkis suspension packages are the SH!& for old muscle cars so I would venture to guess they are quit good for the GTO.