As long as we talk about the same engine choice a (mostly) common engine section / thrust structure would make sense. There are only so many ways to plumb it after all. Since thrust stays the same "just" add a 8.4m adapter ring...
The who gets to be contractor game certainly got messed up by the ULA merger.With all the STS heritage hubbub Centaur-G Prime should have been the some leverage..
How?SLS needs an upper stage, so how about a heritage piece like the SSME or solids.