Not sure GS every used "factory traction bar" GS cars had relocated rear axel bumper brackets. You could box the rear control arms, kits for this are about $35.00 and add a rear sway bar off later GS. From what I remember Rear control arm braces were not used until 68 on GS. Pontiac GTO used them in 67. 68/72 braces are not a direct fit, but you can cut 2' off the straight end and re-drill the hole and use them on 65/7. I did this on my 66. Used 68/72 bars go for about $50.00, new repro are slightly more. Repro 65/67 are about $100.00 from Ames Pontiac. You could also add Poly pushing to some or all the rear control arm bushing. At least put good quality new rubber one's. Be careful some on the rear control arm bushing have a very thin metal jacket, OEM were thicker. Hope this helps.