The size of Upper Stage & payload which the DIVH could loft, if powered by the J-2X, would produce g forces above 8g. That option is not viable.
RL-10 is a great fit for DIVH and if ANY engine upgrade were considered, J-2X should be dropped in favour of RL-60. But even that makes little sense considering that clustered RL-10's can offer the same performance without ever suffering from the high development cost.
Ross.
Yah an upper stage as big as the Ares I's would be too large for Delta IV-H even with the RS68As as it only has 2,106K lbs of thrust.
But interestingly F9-H has almost as much lift off thrust as Ares I.
27x Merlin 1C @ 125K lbs or 3,275K lbs which is even more then the 3CCB Atlas V 2,582K lbs.
I used sealevel thrust numbers vs vac numbers.
Though the standard J2 or 4 RL60s probably would be a closer match for F9-H
The biggest limitation for all three vehicles might the strength of the interstage structure on the center booster core.
The Ares I US is heavier then even the S-IVB so you can't reuse the whole stage.
