vibration after 3.23 to 3.91 swap

I had a very soft & oil soaked transmission-mount in a '64 Newport that would eminate vibrations through the car at certain speed-increases.
Then again, the Newport was also lengthened by 2 feet in the middle and a 2-piece driveshaft was made for it at the time.
But the soft-rubber mounted center supportbearing in combination with an oil soaked transmissionmount made for some serious vibrations in the car.

Took some time to find out by accident. This was at a time I just started wrenching on cars.