Getting a straight-edge in place can be difficult to impossible. I've had good results using a mirror to look over the sheave and long the belt. Any misalignment shows up pretty well. Alighment is very important. Some other things I have learned along the way:

Worn sheaves are bad news. Even if they look OK they might be worn. I don't like pressed steel sheaves on alternators. A good alternator shop should be able to supply machined steel ones.

My belt of choice is Gates Green-Stripe.