a518 Vibration
Mine has a slight vibration in 1 thru 3rd also, real smooth on OD.
My first time I ran this set up it was smooth in all gears. Then I blew up OD, rebuilt whole transmission and then I had the slight vibration. I just got use to it, since never time to pull back out and look.
Interested if you find your issue.
Project`67 Dart GT 340 A500