The above setup should work with a 7sp cassette and 9sp Ergopower levers as the Mavic 840 was - I think - Shimano compatible so a "Shimergo" mix as above ought to do the job. My CX bike had an 840 with 10sp Chorus levers (alloy) and SRAM PG850 8sp rear Shimano-pattern cassette and all worked well. Until the mech ate too much long grass during a race at Hillingdon and died spectacularly (thanks to the Ergal mech anchor bolt ripping out of the rear dropout - its successor is a short-arm Sachs Quarz with a chrome steel bolt made of sterner stuff).
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