Look in the back of a Moog book, or see if you can find the listings online somewhere. Use the equation for a spring to determine the new installed height.
The rear of a wagon is too heavy to work with an F body spring. Always do the math. The back of the spring book provides all of the specs where you can calculate the compressed height of the spring. Remember F=kx, where F is the force (or weight) on the spring, k is the spring constant, and x is the the amount the spring is compressed due to F.
Using the specs for a stock spring, you can calculate F and then determine the compressed height when using another spring.
Marc