What engine do you have? Also keep in mind that the balancer can be cracked and this will cause the belts to go slack. The balancer not only has a bolt to retain it but it's also a press fit and this is vital important. If the balancer slides on loose it works against the bolt until it breaks it, then all kinds of problems occur. WHen this happened to my engine it sheared off a section of the crank and I had to replace it.
As mentioned on the Eagle parts, this hints at a 383 stroker, if it's internally balanced but has a external balancer on it, this is big time problems. If it's a externally balanced crank with a internal balancer this is also big time problems, same goes for the flywheel and flexplate.