Belt tracking wrong

Put new serpentine fanbelt on but it keeps tracking off my harmonic balancer pulley towards the engine. The old bely although noisy tracked true any suggestions was thinking maybe the tension pulley doesn't put enough tension? I really am trying to finish this car its my daughters 17th birthday and I fixed her up a nice 70 duster with 318 that old owner converted the belt system to be a serpentine belt the old owner was going to put a supercharger on it think he had a few rocks in head but he made it new motor but has serpentine belt instead of the old multi belt all the pulley lines square right up with each other it just seems to want to track on the pulley on harmonic Balancer any suggestions