I keep breaking the belt on my 91Z 305 after about 2 months. All the pulleys look fine. I don't hear any squealing, it usually just breaks. What could be causing this?

 

Check the crank [as well as others] thoroughly, when i find this on cars here, it can be coz some heavy hander has wacked a chunk off the crank pulley and damaged the multigroove pulley.
Thus putting a sharp edge on it, which in turn saws through the belt, or deflects it off.
Or your tensioner might be running out of line, check the tension is good enough to keep the belt tight, and make sure its inline with the other pulleys, if not it can deflect the belt off.
You might need an assistant to crank the belt tensioner back and forth while you check it, its a bit safer.

 

If you're breaking belts it seems like it is a damaged/misaligned pulley. If you were simply throwing them, I'd say a seizing accessory.

 

My truck was going through belts and it turned out the be the idler pulley. It was slightly worn at an angle.