Spacers don't really apply for thread on (freewheel) hubs. You just thread a bmx freewheel (
http://harriscyclery.net/itemdetails.cfm?ID=1871) on. You might get lucky and get the chainline to work right out of the box. Or you'll need to redish the wheel or adjust the postion of the chainwheel.
In the above scenario, one might use thin spacers (1-2mm) behind the freewheel to make minor adjustments to the chainline.
The "coasting" ability for a cassette is part of the hub itself (
http://www.sheldonbrown.com/free-k7.html)
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This is Africa, 1943. War spits out its violence overhead and the sandy graveyard swallows it up. Her name is King Nine, B-25, medium bomber, Twelfth Air Force. On a hot, still morning she took off from Tunisia to bomb the southern tip of Italy. An errant piece of flak tore a hole in a wing tank and, like a wounded bird, this is where she landed, not to return on this day, or any other day.