The latest (and best) version of the Spence "Alpine" derailleur conversion uses Campagnolo pulleys between two cages plates. I agree with what's been said about freewheels, if you don't use the 32 tooth rear cog you generally don't need the long cage derailleur. Besides a 32 tooth freewheel generally has big gaps in the gear development, not to mention looking dumb - ATMO.
If you need the low gear, than a triple ring is the way to go if your bike will accomodate it. For my upcoming california coast tour, I'll be using TA chainwheels with 50, 36, and 28 teeth, and a Dura-Ace freewheel of 14 - 26 teeth. This setup DOES require a long cage rear derailleur - th current issue Campy record triple, which is Off Topic here, but a VERY snappy performer, FAR better than the Spence "Alpine", which looks vintage but shifts like a soup sandwich.
Mark Petry
sorry Spence!
Bainbridge Island WA