Sagging 67 needs your input

I agree with oldkoot about getting new springs or a competent rebuild. Aim for the same leaf count and spring rate as your current springs had when new. Mopar SS springs will be stiffer and raise the back of the car a little.

Virtually anything you get is going to ride rougher than what you have because your current springs are fatigued. Adding a leaf/coil over will buy you time, but not fix the general problem.