Originally Posted by luns

I'm not sure what I'm missing, but most of the replies in this thread don't seem to address what I read of the question. rccardr did hit the nail on the head though.

The 7401 shifter pod will fit into a 6-speed Shimano 600EX shift lever (SL-6208). I've test fit this, and also later Dura-Ace pods (I think 9S and 10S) and they fit fine. I believe, though I haven't yet confirmed first-hand, that the cable pull diameter of these levers is identical. If that's correct, the 600EX lever with a Dura-Ace pod is functionally identical to a Dura-Ace lever.

However, you asked specifically about the 7-speed 600 shifter (SL-6400). This was a different design where the lever is captive in the shifter mechanism: there is no removable pod, so you can't vacate the lever to put the 7401 pod inside. So, to answer the question as I understand it: no you can't do it. You'd have to track down a 6208, or 8,9 or 10s (or 7s) Dura-Ace lever to perform the swap.

Your derailleur may work with the alternative cable routing, but look out for the cable sawing into the edge of where it exits the derailleur body to go to the clamp nut. Not that there's much you can do to avoid it if it does happen aside from finding a more compatible set of parts. That's the best you can hope for between your lever and derailleur, aside from just running in friction mode.

Otherwise, you'd either have to replace your shift lever (the pod you looked at, in any of the levers rccardr listed that it'd fit into), or your derailleur (9s Dura Ace should work or any non-Dura-Ace RD).