1966 Other Plymouth Models vibration

I purchsed thic acr in 1668 adn have matained it my self.I took it out of service in 1983 and have just finished the restore.All new front end,brakees,engine mounth,with a engine rebuild that included balancing,also new clutch all new bearings in the mopar 4 speed.The engine is the commando v8 273.My problem is a vibration that starts between 55 and 60 mph,and will increase when going down hill with the clutch in.I have had a new drive shaft made for it,and have tried two sets u-joints.I have changed the angle of the read end by 2 degree and 4 degree going both ways.I am looking for somrthing else to try.As you can see typing is not my strong suite,and I thank you in advance.and have a blessed NEW YEAR.