Search is your friend, this has been covered a number of times here. You may get various opinions on how to address it, but I always reduce the high speed clutch pack orifice like the shift kits do. What you are feeling is the overlap of 2nd band and the high clutch pack. So slowing the timing of the clutch pack engagement is the easiest thing to do after the tranny is in the car. May take a couple of whacks at it to get the balance between a clunk at slow speeds vs flair-up at RPM, but you can get it.