I am curious how you know the Moog springs you are looking at are stiffer if you don't know what the rate is of the springs that are on the car now.
The rate can be calculated if you carefully measure the wire diameter, spring diameter and count up the number of active coils. If we can determine the rate and the free height and the height at ride height (which you have already given) we can then accurately determine the spring load. With that information you should be able to go through a spring catalog and choose one that will give you the height you want at a softer spring rate, even without knowing the weight of the car.
Regards,
Andy
