OEM AC, Carlisle 3 and Dayco HP are all good belts. I have ran all 3; I recently bought a Carlisle Ultimax 4 for about $100 or $120, which is kinda pricey, but wow! Much bigger cogs (both in width and depth) looks like this belt can take a beating (severe duty i suppose). Haven't had a chance to use it yet since i put a brand new AC belt on @ the beginning of the season.
Basically with a 600 if you stick to any of those and don't cheap out with a $40 belt designed for 500 fan cools you should be fine.