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The thing necessary in any spring to get you the same ride height as originally is that the replacement spring exerts the same force on the suspension as did the original spring when the suspension is at the original ride height. For coil springs, if the replacement spring is a little too short when it exerts this same force, it can be shimmed at its ends. If it is a little too long, it needs to be cut shorter, which however gives it a proportionally stiffer behavior in use. The unloaded (or "free") length of a spring is no direct indication of its usefulness.
These principles, and the formulas for calculating a spring's properties and how the spring's leverage on suspension arms is accounted for, are expertly presented in a book by Fred Puhn called "How To Make Your Car Handle", that has been in print for many years, by HP Books and its successor, and is sold in speed shops and bookstores that have a good automotive section. There's no good substitute for accurate measurements and doing the math, then fine tuning the setup once it's assembled. Also, shock absorber stiffness needs to keep pace with changes in spring stiffness.
Charles Greenlaw, Sacramento Calif.
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