72 340 engine in 67 barracuda around 60 mph have a small vibration you feel in the seat not the steering wheel. New 904 Tci Streetmaster trans new 8 3/4 355 Posi rear. New drive shaft also. Wondering what it could be?
Check rear tire balance and runout. Also check the rear drums for the same thing, had that happen to me, brought the drum to the machine shop to be cut and it was discovered that it more was than egged enough(Another F**K YOU china moment).