The Dura Ace RD740x idea works except for the limited capacity of this short cage RD.

It is possible to transplant a longer cage from a from a older MTB RD unto the earlier 7400 RD. The early versions of the 7400 had a bolt on the front like many of Shimano's RD in the early '80s.

This does not overcome the OP's desire for higher end shifters though.