Vibration in driveline or motor/tranny mount ?

Well, I finally got under the car and had a look at the numbers on the engine/tranny.
The casting date on the engine is December 1964, so that fits the build date of January 28th of 65.
The stamp on the tranny was 2466123, which says it's a 65 Dart/Valiant/Barracuda.
Based on these two things, I would have to say the engine/tranny are either original, or at least date correct.
After 65s & Caveman's revelation about the adapter ring if the engine is a later model, I was almost sure it would be that, but now it can't be.
I had a look at the engine mounts while I was under there, and they look fine. I forgot to look at the tranny mount, but I will.

I just find it terribly hard to believe that the ball& trunion or a u-joint could cause this type of vibration. It's more pronounced when in gear, especially the harder you get on it between 25 & 35MPH, but it's also there, although much more slightly, when the car is in Park or Neutral.

I'm going to have a closer look at it next weekend, and see what else I can find. Maybe it could be the harmonic balancer.