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.