Reading an article about new engines for the SLS (Summary: There is no plan what to do once the current SSME's are expended), they mention that the J-2X is too powerful for the upper stage of SLS.
Ultimately, the 294,000-lb.-thrust J-2X engine may be too powerful for the SLS upper stage, at least in the 105,000-ton Block IB intermediate variant en route to the 130-ton version needed for Mars missions. “Essentially, that stage wants to be about [120,000 lb. thrust] total,” May says, and the agency is considering four RL-10s or two “60k-class” engines instead.
The original J-2 was about 234 Klbs of thrust. The J-2X seems to be uprated to 294 Klbs thrust.
Regardless, the J-2 was the second stage engine on Saturn V. There was a cluster of 5 J-2's. The third stage used a single J-2.
So this leaves me wondering, if the upper stage on Saturn V was appropriate at 234 Klbs thrust, why is SLS, which is meant vaguely to be in the same class, more likely to need only 120 Klbs thrust?