I went by torque a few times and 2 out of 3 times the bolt broke. They were not brand new bolts but i expect that the torque im told is correct... anyway i go by feal and then i check to see how close i am on the torque wrench. It works well for me. No broken bolts and no loose bolts.
IMO, use nylock nuts on them and tighten until the nut just contacts the shackle plate. On aftermarket shackles, since it's a "standard bolt" thru the bushings, you should be able to rotate the bolt with your wrench.
I keep mine slightly loose to eliminate friction interference with the shackle plates and bushings....adds a tad bit of flex.
Thanks for all the feedback, based on your replies I'm sure the previous owner overtightened them. I will get some new nylock nuts and do it right this time. Seems like im having to go thru most of the truck that way.....just doing things right ...once.