Been doing a mechanical restoration on my 67 Camaro for the past few years and have replaced the motor, rebuilt the Muncie trany, new custom drive shaft, new clutch and flywheel, rebuilt the 12 bolt posi including new axels & ring and pinion, new wheels and tires. Problem is the car has a harmonic vibration at higher speeds in 4th gear. Have tried adjusting pinion angle with wedges made for this but the problem still exists. The pinion angle didn't look or measure bad to start with but I was trying anything. The motor mounts are new and correct for the car /motor combo. The trans output shaft is about 3 degrees up and the pinion is about 2 degrees down at this time. I had the drive shaft balanced again yesterday just to be sure & have purchased a new pinion yoke today but have not installed it yet. The old one shows some wear but not enough to say its bad but I'm grasping at straws at this point. Would like some thoughts from others on what to check. Really at a loss as far as what to check next. The vibration does not go away when the clutch is pushed in and it comes in around 55 mph in 4th gear. thanks in advance.