Make sure the centering pin in the springs is long enough to locate the shim and backing plate. I've ordered steel shims from Speedway Motors before. Steel lasts longer than aluminum. I removed a set of aluminum 1 inch lowering blocks after several years of use. Corrosion was starting to occur on both clamped ends of the block, and I live where they never salt the roads.
Be sure to clean and wire brush/wheel your u-bolt threads. I use a light coat of Anti-Seize on them too. If the u-bolts are compromised in any way, replace them. Look for the longer nuts that come with them too. I always add grade 8 washers if I don't have the factory thick washers. Recheck the torque on them several times after installing and driving the car. Mine always take 3 or 4 retorqueing sessions until they hold the torque specs.
