I own and operate a chassis and suspension shop. This has been my focus for nearly 20 years in the custom car and truck business. Here is the simplest summary I can offer, and there are volumes more to be sure.
1. Springs are heat treated to set the hardness of the metal. Whether coils, or leafs, heat destroys them. NEVER use a torch anywhere near them. Anyone suggesting this is more than foolish. They are dangerous.
2. Spring rate (stiffness) is determined by number of coils and size of wire they are made from. If you remove wire by cutting a coil, the spring will be stiffer, just as if you shorten a beam it will flex less. If you cut a coil, the increased spring rate will result in a former ride. Cut another, and it will be firmer, still. At some point, it will be too stiff to be comfortable, and can impede proper suspension function. "Too stiff" is a matter of design purpose. A daily driver will be softer than a pro-touring ride.
3. Cutting a coil will lower ride height as if the suspension were compressed by added weight. If over done, steering geometry and suspension geometry are affected in some very bad ways. Bump steer and camber behavior can lead to adverse behavior and unsafe road manners. For this reason, dropped spindles should always be the first choice (suspension and steering are unaffected). IF an increase in spring rate is desired, minor cutting of the coils can be used to fine tune ride height and ride stiffness. One coil generally equals two inches. This should be the most removed. For all the same reasons, dropped springs and dropped control arms should be avoided.
If cutting a coil spring, the best method is a band saw, either stationary or handheld. A SawzAll is too jumpy for most. To accurately match a pair, use the tail removed from one spring to mark the cut on the second.
If an abrasive wheel is the only means available to cut a spring, place it in a bucket of water, with only enough above the water to safely cut the spring. This will keep the heat from moving down the coil.
Remember, 30 year old springs will be a shadow of their former selves. New coils are very affordable from Rock Auto, and others. Keep in mind that they will settle a half inch or more over a few weeks.
Lastly, ride height and drop measurements are always taken by measuring from the fender lip, to the center of nthe spindle, so that tire size and air pressure are not a factor.
