I have a 1988 Ford F-150 6cyl. 4.9ltr - and it keeps chewing up serpentine belts - like 5 so far. I have replaced the alternator, power steering pump, smog pump, a/c pulley, belt tensioner, and the water pump was replaced about 3 years ago. All aforementioned parts came with new bearings.
Problem started when the alternator lost a housing bolt, and I replaced the alternator and belt. And yes, I am using the correct size belt. I'm a motorcycle mechanic for 30+ years now, and I am completely baffled by this. PLEASE HELP

 

Use a square and go from pulley to pulley and make sure one isn't out of alignment. One rib off and the belt is toast.
Check to see if any are obviously seized, some move freely some have resistance, I'm not sure which ones do what though sorry..
I know you said you get the right belt, but there are at least 2 different belts for each truck and different ways to put em on, no offense, but it happens.

 

already squared up all the pulleys, as well as checked the harmonic balancer