How about this to "trick" 8 speed shifters into working on a 7 speed wheel... get an 8 speed cassette and blow the rivets out. Only put the smallest 7 sprockets on your existing freehub (use a spacer on the inside if necessary). Set your limit screws and tune the indexing based on top gear. The spacing will be correct and you'll be locked out of the lowest gear on the right hand side. Cost=cassette and shifters only. NBo worries about freehubs or wheel spacing.

Or get these: http://www.velo-orange.com/diacosibash.html :-)