Try to stay away from really long or double shackles, they don't work and are unstable. put the weight of you truck on the spring so it flex's out.Then measure the eye to eye length.Take that length and use your shackle to get the degree you want on the mounting point of the frame. If you measure the length of the spring following the curve it will tell you how much that spring will grow when compressed. you can then measure that out to show you the compressed angle of your shackle. Tack the mount in place before welding it and see how it measures out with weight on it. Hope this makes some sense.
