Shackles are a bad, bad, bad, bad, bad, bad idea. They add stress to the spring mounting points that they were never designed to handle (one day you'll hit a bump and your new shackes will be in the trunk). They screw up the handling of the car. They will eventually bend your main leaf over backwards. They make your gas tank an inviting target for the next Corvette that rearends you. I lived through the 70s and the ultra-wide rear wheels that stuck out past the fenderwells and I saw all of these things happen. Don't do it.
Oh, and air shocks are an even WORSE solution. Don't get me started on that topic.