Need suggestions on vibration issue

check the alignment of the bell housing with the crank (tranny drive pinion bearing retainer hole) Mopars are notorious for having to have this dead on spec.If out of spec you will get a vibration.I had someone bring me the same car as yours with same problem.They tried everything before asking me even tearing the motor down and balancing the rotating assembly.I checked the alignment which was out of spec made adjustments and problem went away.I may be wrong but I believe all SB balancers and flywheels are the same being that they are either for internal or external balance not motor size.