4 speed problem

I have a 1969 gts 383 4spd car, recently done a rotisserie restoration including having the tranny gone through, new clutch and pressure plate. Now the problem is that when I start out from a dead stop in 1st the car tends to shack and shimmy til I feather the clutch out and give gradually accelerate. It only happens when I am in 1st, are there any suggestions or advice to my problem?Thanks Bob.