if you are staying with v belts, maybe a simple belt "retainer" made from 1/2" round stock with some flat stock welded to the end, then bolted to the alternator and water pump approximately 1/8" above the pulley may help retain the belts when they stretch at high rpm.
i have seen this simple idea on several applications. it seems to do the job, so i have stored this idea if i ever need to use it.
beer