The fully forged lowers are 8-lug parts. I think usage on the 6-lug trucks is as simple as swapping it in place. I don't think the taper in the knuckle has to be modified unless swapping in GMT-800 knuckles.
Three kinds of lower control arms:
1. Entirely stamped-steel. Bolt-in ball joint.
2. Stamped steel, forged outer end. Press-in ball joint.
3. Forged control arm. Press-in ball joint.
They're all interchangeable as an assembly, although I don't promise that the ball joint tapers are the same (they probably are.) The general consensus is that the fully-forged arm is the most desirable.
Just checking: Is the torsion bar "hex" receiver the same size on all three? I started to wonder if the 8-lug forged arms weren't different / bigger there.