I would have thought try a narrower belt so its sits deeper in the pulley. My brothers 451 kept tossing belts. We tried a narrower belt problem solved.
Before the belt flew off, I had never noticed that the belt was twisted because of pulley misalignment and a poorly machined water pump pulley that was thicker on one side of the groove than the other. Only after I really looked at it did I see it was crooked. I spent a lot of time lining up the pulleys and getting them as close to perfect as possible.
Serp belt has more area and flatter grooves, just seems like it would probably be more likely to stay put at 7 grand.