I believe I have the same problem on my 5.7 OSI. I have changed the tensioner (normal first step), idler (pully where the belt visibly walks out), and power steering pully (area where belt dust is most prominent). At this point I'm running out of pullies. The belt walks outward on the idler, which is just before the alternator, so I am thinking it must be the alternator. Is there any way to adjust the position of the alternator pully? I believe it needs to go in, which theoretically should stop causing the belt to walk out on the idler pully, and stop getting tore up on the PS pully.
I have a 5.0 GXi-E if your alternator bracket is the same as mine I believe it is a design error by VP. I machined the bracket to correct the situation, no more belt failures or piles of belt dust. First thing is to find out just how far alignment is out. The front flange thickness of the crank, power steering pump and alternator pulleys are not the same so care has to be taken to get the correct measurements The way I did it was :
1. Place a straight edge across the crankshaft pulley and with a caliper measure the distance from the straight edge to the first peak between the first and second groove of this pulley. This is your datum measurement.
2. Then holding straight edge across the crank pulley in line with each of the other pulleys in turn, measure the same distance from the straight edge to the first peak. This will tell you true position of the pulleys to each other.
3. Check the pulleys for being parallel to crank pulley. Place a straight edge, in turn, across the alternator and power steering pulleys and measure gap on near and far sides of the crankshaft pulley. Difference in the gap of either side of crank pulley indicates the degree the pulley being measured is out of parallel. Then do the same between the alternator and power steering pump.
With this information I found that my bracket was 1/4" too far forward (alternator cannot be moved forward or back). The bracket was at a very slight angle and the power steering pump pulley was slightly too far forward.
However, the belt tensioner was in the correct position relative to the crank pulley! I can't imagine how the designer of this bracket got it so wrong!
To correct the situation I machined the aluminum bracket bosses that the mounting bolts pass through. I removed 1/4" from each of the three bosses on the back of the bracket. I installed the bracket and alternator and checked alignment, I found it needed one S.S. washer between the engine and the bracket at the lower bolt to correct for parallel error. I then had to make a 1/4" spacer to put under the tensioner to bring it back to the correct position and it needed a longer bolt. Checking the power steering pump showed it was now 1/8" too far aft. I had to buy a PS pulley puller tool to move it into alignment.
Quite an ordeal, but now I don't worry about belt failure or cleaning up all the belt dust.