This is from the Duramax Forum regarding 4" and 6" spindle lifts:
When you replace them, make sure to get the full replacement upper control arm, so that the ball joint is set at the factory and you get new UCA bushings too. I know on the 1500's, I have seen too many guys just press in the new ball joint into the old control arm, and for one reason or another they sometimes pop back out. If you are doing it yourself, it always helps to take the cam bolts to a wire wheel and some anti-sieze so that they don't sieze inside the UCA bushings.