Bob's Resource Website (2007)
Adding a leaf spring to remedy a sagging rear end....
You'll need a couple floor jacks, big one and a small one. A good cutting tool to carve the springs up
If you can obtain a couple long leaves from an old leaf pack, you can use the longer 2 leaves and trim each one back to a size that allows an equal size on each side of the car. Alternatively, Ray Fichthorn (Studeman@triad.rr.com) can provide you with a service pack of 4 springs fairly cheaply. You may still have to trim them to fit your preference.
Loosen the rear wheel lugnuts. Jack the rear of the car up by the center of the axle until the wheels are about 4 inches off the floor. Place jackstands under the axle, midpoint between the center pot and spring perch. You may have to use a couple pieces of "2X12" to raise the jackstands enough. Take this time to crawl under and spray the bottom plate nuts, shock and sway bar attachment points with a good dose of PB Blaster to prep them for removal. Remove the wheels. Place/build another support under the forward leaf spring mount and shim it sufficiently to meet the current height. This will support the weight off the car when the springs are unbolted from the axle.
Loosen /remove the shock absorber. Loosen / remove the sway bar link. At first, only LOOSEN the nuts on the U-bolts that attach the spring to the axle. The springs will start to drop, so, if you have another small jack, place the pad under the springs, close to the mount, to support the one side, while you remove the components. Remove the 4 U-bolt nuts and the bottom plate will drop off.
The weight of the spring force is NOW on your small jack. Dropping the jack a few inches will relieve the force. You can, then, clamp the rounded head of the bolt holding the springs together, with a vice grip and remove the nut. This will cause the lower section of the springs to separate. The main leaf is still attacjhed to the shackle mounts. The upper 2 or 3 are clamped to each other.
At this point, you make a decision how much spring you want to add. The next part is probably the hardest, depending on your tool collection. You have to hack off the ends of the single leaf to meet your specs. I did it with a Sawz-all, using up a bunch of blades. Ray says you can use a cut-off wheel. If you use a torch, you might damage the temper of the spring if they get too hot. A hack saw is the long route....
This is an energy intensive repair, with all the manipulation of jacks and hopping up and down, but a little patience goes a long way. It really is a good, inexpensive repair, thanks to Ray Fichthorn.
Personally, I added a 40" section to the rear of an Avanti, whose rear leaves actually arched downward about 2 inches with the weight of the car on them. The added 40" leave raised the rear of the car, so it arched upwards 2 inches, with the weight on them. On a 4 door, LWB sedan, I added a similar size leaf but didn't measure it. It fit between the 2nd and 3rd factory leaves. The length was half the distance between the 2 factory leaves. Just these 2 alone, raised the car about 2.5 inches. The fender skirt used to cut across the top third of the gold center in the hubcap, now the skirt is about 2+ " above the center spot.