If it were me I would get a set of strutbags from AIM Industries, a small compressor like a Viair 380, a 2-3 gal flat round tank that can mount in the spare tire well, and 4 valves. You'll only need 1/4" or 3/8" line and 1 switch to control the whole rear as one. The reason I would go with this over a set of custom made springs is that once those springs are made that's what you've got. If you ever make any changes that adds more weight or takes some away how will those springs ride? Not only that, you're going to have to go with such a high spring rate to compensate for the weight that it's probably going to ride really stiff and is going to put more stress on the strut towers and unibody.