What hubs are you running again? Some older Shimano hubs were 7 speed only; 7 speed cassettes are a bit narrower than 8 speed ones. You can run 7 speed cassettes on 8/9 hubs by using a 4.5mm spacer. I'm doing this. Shimano derailleurs don't care what speed they are, so you can use a "9 speed" one with 7 speeds at the back. I'm actually doing this too
How many speeds do your thumbies have? That determines how you proceed from here...