I take it that you have replaced the factory shackle pins with the aftermarket style that has the nuts on the outsides of the shackle plates. The manual does not give a specification other than to "tighten the center bolt securely". That said, I'm going to give my opinion on what to torque it to, based on my experience as a Suspension/Brake/Alignment mechanic. The ends of the replacement pins are 1/2"-20 thread as I recall. I would go with about 60-70 ft lbs to ensure proper seating of the tapers.
If you are using the modern rubber style bushings, use the same torque value, and make sure to tighten them with the weight of the vehicle on the springs, as that will prevent bushing bind.
