1. Yanmars are notorious for having pressed steel sheeves on the FWC pump. These sheeves rapidly pit and rust and when so quickly abrade belts. Check the surface of vee faces on the FWC pump.
2. If you have a 'smart regulator' you can derate the 'ramp-up' (how FAST the alternator is energized). If the 'ramp up' is too severe the belt can momentarily slip and t herefore 'abrade'
3. Alignment but at such large sheeve center to center distances, v belts 'usually' can handle such misalignment -- farm equipment has lots of misalignment, depending on the characteristics of the vee belt (but needs smooth surfaces on the faces of the sheeves see #1).
4. 90-100 Amps is about the upper limit of power transmission that a single 1/2" vee belt can handle (stretch and slip). Get a belt tensioner (tool) from an automotive supply (NAPA, etc.) so the the belt tension is 'perfect'. If belt tension IS the problem, consider to applying a spring loaded automatic belt tensioner - Grainger, etc. stuff.
5. With a 'smart regulator, you can derate (slightly) the alternator.
6. Gates are the absolute VERY best vee belts, so its NOT the quality of the belt. If stretch due to max. power load on the vee is the cause, get a Gates with a 'spectra' core.

#1 & #2 are the most probable 'culprits'.