Put a straight edge on the alternator pulley. My bet is that its centerline is off in at least one plane. Or the pulley centerlines are forward or back from one another. The straightedge should stay perfectly square to both the crank and and water pump when placed on the alternator pulley. If the straightedge doesnt touch both faces of all three pulleys you have an issue. Do the same with the crank and water pump. I've had combos throw belts. Every time it's been one of 2 issues. Alignment or bracket flex. Currently have 3 cars. All have modified brackets, pulleys, etc. None of them ever through a belt. All run to at least 7000.
Doug