If you want to use an screw-on freewheel with a 8 speed indexing system, they work fine, as long as you specify the cog-to-cog distance. You wont fit in the same number of cogs, ie you may have a 7 speed system, spaced for Campy 8 speed indexing.

Campy 8speed is 5mm,
Shimano 8spd is 4.8

In the UK we have a freewheel specialist at Highpath Engineering.
http://www.argonet.co.uk/highpath/cy...aqs.htm#modern

He builds up freewheels any way you want.