Dura-Ace RD-7400 requires Dura-Ace 7400 series 6sp shifters.  They do not work properly with any other shimano 6sp shifter unless an alternate cable routing is used.  See SheldonBrown.com for details on this alternate routing.

Shimano's top of the line race quality component group in the mid 80's through the early 90's. The 7400 series Dura-Ace group saw several interations including the introduction of 6 then 7 and finally 8 speed SIS shifting. Eventually upgraded with 7410 parts before finally being completely replaced with 7700.

Dura-Ace was developed to give the professional racing cyclist a fraction-of-a-second advantage that can spell the difference between victory and defeat. Advanced functions like 8 speed SIS, Super SLR brakes and Shimano Total Integration provide the most refined feeling of control available in a group today. Ultra high-precision mechanisms, quality materials and rugged construction assure that Dura-Ace takes all the punishment that pro-level road racing can dish out. (1991 Dealer Manual).