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I'm still pretty confused about the best course of action in my situation. My frame has a rear spacing of 126mm and my hubs are 126mm width Shimano 6 speed Uniglide 36/36. I would like to use as much of the Shimano 600 Ultegra "tricolour" groupset as possible and I'm not sure whether this will be possible with a 6 speed hub as the 6400 Derailleur is 7 speed (velobase.com/ViewComponent.aspx?ID=c7069264-613f-48e1-8112-d3236e2fd096&Enum=108). However this build (the same frame as mine) vitus979.com/my-vitus-979/ seems to have used that derailleur on a 126mm width hub.
I'm afraid so; I'd wait to see if it works before you thank me...
Yes, 7 speed freewheels (almost) all have the same centre-to-centre spacing: http://sheldonbrown.com/cribsheet-spacing.html