Originally Posted by
lobo241
What size spacer would I need?
It depends. If you have 600 "Tricolor" 7-speed (assuming I'm not hallucinating) indexing DT shifters, and you want to use indexing, you'll need an 8-speed cassette spacer. You'll put the spacer on, then the 7-speed cassette, then the lockring. If you don't have indexing then you don't have to care: just use an 8 or 9 speed cassette and chain, and be happy (after verifying/modifying your drop-out spacing). Some people don't care for friction shifting on 9-speed; I'm not one of them. I find that it works well as long as you shift just as we did back before dirt was invented: ease of on the pedals whenever you shift.
Good luck, and enjoy your new/old bike. Updating drive trains does wonders for these old machines.