You can get the theoretical shackle length by measuring the spring along the main leaf, following the arch, center of eye to center of eye. Then measure from the front spring mount that far back and to a point 1.25" below the frame. Then measure from that point to the center of the upper shackle mount eye, now you have the longest length the shackle "can" need to be. (shorter would hit frame, longer may add more stress to frame)
I say "can" because this length is only if the spring is designed to flex completely flat. That may not be possible without permanently damaging the spring. Speak with the spring supplier for the maximum flex.
Edited for more clarity..............