Originally Posted by
xenologer
7 and 8 spd spacing are close enough that your 8spd shifter might work as is.
if not, rather than mess with spacers,
you might try fudging the cable clamp on the derailer to very slightly alter the cable pull ratio
ie. clamping the cable slightly inboard/outboard of the groove to make the derailer swing less/more per indexing detent.
If I understand this correctly it is a modification of the alternate cable routing that is needed to run pre-9speed Dura Ace shifters with a non-dura Ace rear derailleur. The alternate routing moves the derailleur more than the normal cable routing. I am running a 910 XTR rear deraileur with Dura Ace levers and it works great with the alternate.
The same principle can be applied to a 8 or 9 speed shifter to a 7 or 8 speed cassette. I have always wanted to test it out, but It would require less cable pull than the alternate cable routing. My gut thinks that drilling a hold through the drailleur attach bolt and run the cable through it like a vintage brake attach might be close, but I haven't actually tried it.
John