How much lift will the stock lifters handle?

Retainer to guide clearance will limit you somewhere under about .550" lift at the valve if the guides are not getting cut down. The 5.2 and 5.9 Magnums use the same heads. Stock factory lifters can handle any street hydraulic roller that will fit in that head constraint.
I never measured piston to valve clearance on the 5.2 Magnum yet.