If it is known the wire diameter, coil size. number of turns, free height and compressed height the rate of the current spring can be calculated and then we can compare the new springs to see if they are softer or stiffer than what you have now. but there is light becuase there are companies that do all this for you. Here is a list.
Coil Spring Manufacturers Serving Pennsylvania (PA) - Zycon
some of those guys do automotive and will be happy to make you just the spring that you need.. I think it was another guy that had the same issue and we had him take a look see at his shock valving because a shock tha tis valved for a heavy car may be too stiff for a light car..
Maybe you just need to bite the bullet and get some coil overs from pro or someone like that, that are adjustable so you can get closer to the ride an handling that you are looking for.