Add-a-leafs are designed to provide an additional 1.5" to 2" of height to your factory leaf springs. The exact amount of lift will depend on the weight of the engine or other accessories that have been added. They will also increase your load capacity.
Add-a-leafs come in different lengths. Short add-a-leafs are simply added to the existing spring pack. Long add-a-leafs are inserted within the spring pack. While the addition of any add-a-leaf will firm up your ride, the longer add-a-leafs have the least impact on firmness.
Typically, the addition of an add-a-leaf does not require longer u-bolts. However, replacing older u-bolts is highly recommended.