chevy1500z71

ok, so i have the belt that came on my truck and it is what whipple recomends for the truck, i also have a new tensioner, and all my pulleys are good. but i do have a smaller pulley on the blower. my problem is the belt will not stay tight, it stretches when i smack the gas in neutral and the tensioner goes all the way up and then the belt hops up and moves back a groove on the blower pulley and tears itself up. so i got one size smaller belt, which was way too small(couldnt even get it on, i could have squeezed it on, but why bother if its just going to wear out bearing on something or another), went back to the belt i had and got a bigger idler pulley for the one directly next to the blower pulley, to make up the diference of the smaller pulley. and siting there not running, this setup was perfect, it put the tensioner just right. but once i fired the truck up and nailed the gas, the belt stretches out and does the same thing, not nearly as bad, but same problem.

the problem occurs when the blower makes boost, i guess this is putting the strain on the belt and stretching it. it is a dayco polly cog, is there a stiffer belt out there? one that wont stretch?

i dont know what to do about this problem? am i just making too much boost for a serpentine belt driven setup, it is setup for 6 pounds of boost and im making about 12.