In the past when I've needed to make up the correct length spacer, I've just screwed an extra long bolt of the right diameter/threads into the correct bolt hole on the head after sliding the alternator on, then straight edged the pulleys, and measured the actual distance from the head's mounting face to the rear of the alternator mounting boss.
I've also tried using tubing for making these spacers before with little luck, although if you put a thick walled washer at each end of the spacer that hugs the bolt shaft closely, you might get away with it. They turn out better if you make them up out of black pipe nipples. The best were made by buying (even at 45 cents per pound, the material cost is minimal) solid rod stock from the local used steel yard, slightly longer than what thought I needed, and of a large enough diameter to give a 1/4" to 3/8" wall thickness. I just needed to borrow my neighbor's small (and old) Craftsman lathe to drill the correct diameter hole for the mounting bolt and get the ends perpendicular to the drilled hole.
You didn't give us much information to go on, but hopefully you have the rest of the bracketry, and don't try running without the one that triangulates the end of the mounting bolt to the water pump, especially if you don't get the spacer's ends perfectly square (perpendicular).
