i have a vibration from about 2000rpms to about 4000, its not very bad, just aggravating. i just put in a 351 and i didnt mark off the driveshaft bolts location, and i didnt measure the angle of the transmission(if that matters). also it has a stock harmonic balancer (1984 motor). i was thinking transmission, but could that be symptoms of a bad h.b.? i have an aluminum d.s. and was thinking that it could also be out of balance maybe?