crazy vibration

hey guys i just rebuilt my engine and put a new clutch in my 68 cuda 383 4 speed. my question is when i let the clutch from a dead stop i have a crazy vibration / banging coming from i think is the rear suspention. i thought it was the tranny mount (it was kinda split) but after installing a new mount i still have it. once the the clutch pedel is out there is no noise. the car is a fomula S factory 4 speed any thoughts?