I'm no expert but I know that several lengths were used (or was that fan spacers - I forget).

Anyway, you want a spacer that will align the alternator pulley with the other pulleys and keep the alternator pulley "square" to the belt.  I found the length by putting the upper bolt in through the alternator and into the block without a spacer.  Then I attached the alternator to the lower pivot mount.  String the belt over the alternator and other pulleys, then cinch the nut down on the alternator adjuster so that the belt is sort of snug (you don't want it too snug because it will make the next steps much harder).  

Either use your eye looking down the belt or put a straight-edge across the alternator pulley and move the top of the alternator bit by bit until it's in the same plane as the other pulleys, then measure the distance between the block and the alternator body to get the correct length.  I remember using a piece of black pipe as the initial spacer just to make sure my measurement was good before cutting the "real" spacer to the final length.  I can't see any reason why you could not just cut a short length of black pipe and use that with a bunch of washers to find out the dimension as well.

As John said I guess we typing the same time.