have a look at Highpath engineering for cog spacing:
http://www.argonet.co.uk/highpath/cycle_/faqs.htm#index

the bad news is that Shimano 7 speed is not compatable with Shimano 8 speed (4.8mm to 5mm cog spacing).
The good news is that it is compatable with Campagnolo 8speed.
Get yourself an old Campy ergolever and derailler, and you can use your current 7speed cogsets.
Alternativeley, use Marchisio cogs and spacers to select the cog spacing you want (eg 4.8 or narrower)