Art,
I used a pretty unsophisticated way to apply tension. I first made a mark on the upper mounting bracket where the bolt was on the old alternator. Then when I installed the new alternator, I simply grabbed hold of the alternator with my left hand and pulled hard, making sure I got to the mark or a little further. With my other left hand, I tightened the bolt. I checked the play in the belt across the top. The span is a little less than 1 foot, and I've got about 3/8" of play.
I have attached links to some pictures below:
Alternator Pic 1
Alternator Pic 2
Alternator Pic 3