the spring rate is whatever is needed to just hold the car up

2 things are missing:

1. there's more than one way to get the car to the same ride height (assuming you know what you want already):
A. short stiff spring - stops transferring weight when it tops out
B. long weak spring, the extra length is pre-load - transfers weight up to its full length

2. spring stiffness must be enough to control the weight of the unsprung components (axle etc.). An 8-1/4" and a Dana need different rear springs regardless of the rest of the tuneup.