V-Belts run on a few principles.
Look at the side view of a V-belt.
The cord body you see is where the power is transfered.
The rubber V below the cord body is there to keep the belt from being forced inot the pulley groove.
The rubber above the cord body is there to give substance and stability to the cord body.
V-Belts squeal because they are slipping...I know. EVERYBODY knows that.
But did you know the slip can come from worn pulleys, worn belts, dry belts, overheated belts, wrong sized belt for the pulley groove depth and groove angle, improper tension, belt whip, contamination in the air or directly on the belt.
The belt size, and pulley diameter/contact area play a big role in the amount of power a belt can transfer.
With your one wire alternator keep in mind it was probably designed for use with a poly-V Belt rather than a single V-Belt.
The poly-V Belt is capable of transfering much more torque than a single V-Belt and often poly-V pulley is designed smaller in diameter too. This often results in a smallish single V pulley being installed on the alternator in order to keep the same speed as the original intended use...and this often does not correlate to a good fit with the FE installation.
Worn pulleys get a bulge cut into them where the cord body of the V-Belt rides. Once this worn area is deep enough (does not look like much wear) the new V-Belt can not transfer proper torque because the cord body is not in proper contact with the pulley.
The belt will slip, squeal, and wear quickly but then a new issue pops up.
The belt is no longer a linear sided V-belt. Instead it now has a weird bulge shape to match the pulley and this never allows for full power transfer...It also causes other issues.
If the pulley has the correct depth and angle, and is not worn.
If the belt is the correct size (V shape, angle and depth) and riding at the correct depth in the pulley. (the V-Belt should be almost flush with the top of the pulley, or slightly recessed in the pulley, not sticking up any amount above the pulley.)
If the contact area around the pulley is maximized for the application and location and you still get belt slip...Then you need to change to more pulleys in order to transfer the required power.