69 Barracuda Clutch Shake Help Needed.

IMO,only two sources could be your issue; the clutch or a dancing pinion angle.
A pinion angle by itself and not dancing,has to be pretty lousy to be noticed on take-off. All other sources of vibration would be continuous. The only source for a changing pinion angle is the rear springs.I would get someone to eyeball them while you do the E-brake test .