Gettin that sweet negative arch bro!
If your shackles point down, factory style, then yes a longer shackle should yield slightly more droop. If you have a spring that can utilize the extra motion of the longer shackle, which stock really cant, so youll probably pull the same travel numbers just at a higher center of gravity.
If you have flipped shackles with stock leaves and try to use it like a deaver set up in the desert it will negatively arch the springs because theyre not designed to flex in those ways. Adding a longer shackle to this equation will cause binding if left in the stock location and wont increase the droop, or if you use a drop hanger and move it back your only going to add more stress to the stock spring. The rule of thumb is to use the shortest shackle you can get away with because it will cycle from sprung to droop faster then a long shackle which requires more time of movement to make its radius in the same motion.
