Originally Posted by Clem von Jones

You'll need a freehub cassette and depending on the spacing of your frame in back it might only fit a 7 or 8 speed. Google Sheldon Brown's webpage about it: Freewheel or Cassette?

An 84 will likely fit an 8 speed so a standard 130mm rear wheel would work, but if it doesn't then you'll have to swap the existing freehub for a shorter one and use a 7 speed cassette. You'll only know for certain after you try. It's safest to use Shimano hubs because the old shorter freehub part is readily available if you have to go that route and it's compatible with modern shimano rear hubs and modern Shimano 7 speed cassettes. There's another shorter version of the Shimano freehub called the Uniglide and these are throw-aways. Your bike probably already has that. http://sheldonbrown.com/k7.html

Originally Posted by wrk101

A lot of 600EX were Uniglide, which is totally obsolete. Pull it off and you will know for sure.

I already answered the other question.

Need to give a lot more information on the wheels you plan to buy. You can still buy a freewheel style rear wheel. Or you can buy a free hub wheel (cassette). You can buy a freewheel wheel for a five speed freewheel, a 6 or 7 speed freewheel. You can buy a free hub wheel for a 6 or 7 speed cassette, or a 8/9/10 speed cassette. You can buy a road wheel with 126mm rear spacing, 130mm rear spacing, or 135mm rear spacing. You might even be able to find one with 120mm spacing.

Some of the cheaper new wheels are freewheel style.

To make it even more confusing, I recently bought a freewheel style rear wheel with 135mm spacing (very unusual).