Tom,
If I remember, they tried to come up with ways to recover the SSME's of the Shuttle C, but always with expensive outcomes. Now, using 2 RS-68's, I think we have the chance to build a launcher that while not cheap, would at least start to reduce the cost of launching large payloads. It would require an upper stage, but the Japanese H2-a might fit the bill, and you could drop the engines with the ET so the final sprint to orbit be quite efficient.

I also think that if there ever was support for Mars or Lunar development that a larger vehicle could be constructed from available engines and tanks, with only new thrust structures and plumbing required.
Think of two stage-zero boosters each powered by 4 RS-68's, supporting a core with 4 more RS-68's. With an SSME upper stage it would weigh less than Saturn-5 with more liftoff thrust. While seemingly a lot of engines, the S-5 stack required 11 to complete the job and this would require 13.