Barracuda spring sag in rear.

My '66 was sagging terribly, with huge shackles even, so I went with a new set from ESPO Springs N Things. +1" ride height. I wish I would have done the +2". I have 15" wheels on my '66. I gained 2.5" over the sagging height, and got rid of the shackles. They custom build you a set based on your car specs.