I suggest that you determine what length you need to properly work with the lift springs. You want a shackle that would lay almost flat if and when the spring changes from concave to convex shape (goes flat). The shackle shouldn't bind during that cycle.
To determine the length, take some string. Go along the edge of the main leaf from eye to eye. Tie a knot to mark the eyes. Then pull the string tight and this will tell you where the shackle eye will be when the spring is flat. Measure the shackle length from the stock mount. If you can't find that length shackle, you don't want a shorter shackle. Slightly longer is the better choice.
Once you determine the flat length, make sure it'll work for the extension. Take the string once again. Locate the center pin and mark it's location on the string (knot). Then pull the string down under the axle and make it droop. The backend of the spring will not be straight like the string as it's a metal spring and has resistance. But you should get the idea. The shackle should not be in direct line with the string to the center pin. If it is, there is potential the shackle will compress against the frame and bend the spring. You need to have the shackle forward of the spring to center pin.
A shackle's purpose is to lengthen and shorten the length of the leaf spring during suspension cycles. Hope this helps.
