Want exact ? Four point scale the car with normal load (passengers ,full fuel tank, etc ) compare the results to Eaton spring recommendations. Rule of thumb an aluminum headed big block=cast iron headed small block ,all things being equal with accessories for street. Don't have access to a 4 wheel scale? Give them a call and see what they say.
Spring rate is very important (front, rear) based on what you are doing with the car. Finding load and demand is a must in determining rate for your particular application.
QA-1 tech is another great source in determining the proper spring for your build.