340 Vibration

Looking for a little wisdom. I bought a 1970 Dodge Dart Swinger and the gentleman said he had the engine rebuilt. Don't know how long ago, but it seems pretty stock. I finally got it to run and at idle it is smooth as silk. However, as soon as I give it gas there is a vibration. I thought at first, it was in the drive line, but I can sit in the garage and slowly rev it and the vibration is there. It doesn't have a miss, but when you drive it down the highway, you can feel it in the car. Could it be as simple as a vibration damper, or do you think I should take it apart and see if there is some imbalance inside? Any info you can offer would be appreciated.