From what I remember from the last time I changed those belts there is a section in the manual that describes how you would measure the belt tension without a gates device. The long sections of belt should deflect around 1/2" and the short sections should only deflect about 1/4". They give specific lengths, but I'm pretty sure this method should work just fine. They also tell you to tighten the tensioner on the alternator a small bit after you have tightened all of the other bolts. This keeps tension on the bolt, holding it in place. Other people have said they have to continuously tighten the alternator belt, but this hasn't been the case so far.

My ac compressor was in a different place or I'd give you some advice. I think I had to use a piece of softwood to brace and push against the compressor to get the belt on though.