I think a 740x series derailleur can be run 6 or 7 speed as long as you have the Dura-Ace freewheel and shifters to match. I'm pretty sure you'll be looking for DT shifters; pretty sure they didn't make 7 speed Dura-Ace brifters.

http://www.sheldonbrown.com/dura-ace.html

on a recent build I wanted Dura-Ace hubs but didn't want to deal with the rest of the compatibility issues with Dura-Ace. So I just threaded a Sun Race 7 speed Freewheel on and used Ultegra components.