A kinda common problem with them from what I read. I know the flywheel is obsolete from GM. The LT1 is externally ballanced, so that flywheel is weighted . I never seen one, but if the weight is tacked on then possibly it flew off, thus the vibration ? Jobber flywheels are available. Possible the pressure plate something happened to it and it can cause the vibration ? You may have to take it all apart and inspect the whole clutch assembly to see what shape it all is in.