6 speed to 8 speed

That won't work. 8 speed is wider (or at least 8 speed shimano cassettes are wider than 6 speed), you can use a spacer to fit a 6 speed cassette to an 8 speed freehub but the spacing will still be set for 6 speed and so, 8 speed shifters won't index properly.

Yep, sorry, that's what I meant. 6sp will go on with spacers, and it'll only work with friction or 6sp indexed.