Couldn't we use a coil over set up and remove the leaf all together?
Here is one for the Caddy side of the house.
http://www.streetbeatcustoms.com/Strutmasters/Coil-Overs/Cadillac-Eldorado-Strutmasters-Rear-Coil-Over-Shock-Conversion-Kit-_-CAD_R1/787315/
I am pretty sure these would be a bolt on. You may have to install the leveling kit for it, but that is easy enough and can be programmed in to the system.