Torque them up as described by the previous replies, then do it again at least 2 more times.
Seems like the dang things stretch a bit with use.
I got a couple of turns on each after a few miles.I had new CCOT U-bolts and springs.
I asked a similar question when I did my 4" Skyjacker lift. In the end I went with 90 ft/lbs. As nothing stripped, I'm assuming that the nuts & bolts are happy with it.
As Woody stated, re-torque them repeatedly. After my installation, I drove about 5 miles, and the next morning I ended up putting several turns on each & every nut. Since then, I have re-torqued them at least 6 different times. One thing that I can say is that each time I get a little less movement before the torque wrench clicks.