I wouldn't even mess with stock application springs. With those you have no control over spring rate.
Use coil-over coils. You can get them in a variety of different spring rates, and as long as you haven't marked them most vendors will exchange them for different rates should you not get exactly what you wanted on the first try. There are commonly available springs for 2.0" c-o bodies, 2.5" c-o bodies, and 3.0" c-o bodies. If you expect to need a really long coil then use springs for one of the bigger c-o bodies as they will be a larger OD.