Here is the best fix for sagging springs that I have found. Hellwig helper springs. They are normally mounted under the springs, but I use them over so that they are not really visible. It takes a couple of small mods to the clamps to use them this way, but not much. With the helper springs, you can adjust the height that you want including side to side. There is not much change to the ride. And there is no air pressure to have to maintain. All for under $100.

After years of messing with air shocks, new springs, re-arched springs, etc, I tried helper springs 25 years ago and have had them on numerous cars since. The ones on my Cuda have been there almost 20 years themselves. I have adjusted them a time or two over the years, primarily due to different tires. But other than that, there has been no maintenance. Try that with air shocks.

Why guys go to air shocks that can bleed off or collapse, or go to all the trouble to re-arch springs that may or may not end up putting the car at the height they want is beyond me. The helpers are a much better and easier fix.