So you need new coils for that muscle car? There's a few things you should keep in mind.

1. Is this a original resto?
2. Have you modified the car?
3. What are you looking for in the ride / performance department?
The coil springs on your car were speced by the OEM supplier according to several variables:
1.Model
2.Engine size
3.Air Conditioning
So all of these need to be taken into consideration when choosing your replacement coil springs.
Lets assume the simplest of the above, an original resto. If you have a 68 Chevelle with a small block and no AC specify all of these when buying your new springs. Any parts man worth his salt will be able to get you the proper springs (I like Moog) with that info. This will give you the factory ride height and handling, as would be expected with a resto.
Now what if you modded the car? Say you have a 454 in that 68 Chevelle now. Well at that point I would consider a big block (they did have the 396 in '68) spring for the car. The extra spring load and rate will improve the handling with the heavier engine.
Now what if you want to lower or raise the car? Well here you're pretty much on your own. To raise the rear of an A-body GM simply use the El Camino springs from the same year. To lower the car go with a proper aftermarket spring, these should be properly engineered to maintain the ride quality of the car. I never suggest cutting coils.
Hotchkis makes a nice product if you want to drop your car.
Finally is the situation where the spring is no longer available (NLA) for your car. 1969 GTO front coils are an example of a spring that is NLA from Moog. What then? Try to find the specs for the springs that were originally on your car. Then find a similar spring. 69 LeMans springs happen to be a good alternative for a 69 GTO.
OK, thats it for now on coils! Another post will tackle the rears, variable vs. constant rates.