Not a problem on the belt stretch
Although the longer belt will be expensive, there will be no stretch issues what so ever concerning the belt length due to your high mounting position. Your alignment or squareness on the alt to the water pump and drive pulley is critical on gilmer type belts to keep them on there pulleys. Do not tighten it like a V belt. I run them with just enough tension to keep them on the pulleys and not much more, however the longer the distance between pulleys the tighter you may have to have it to keep it running smooth. A one inch wide belt would be plenty to turn your water pump and alt. Looks like you are set up for a blower belt, should be plenty strong.
Bruce