Couple other things to try to isolate.
Take a half empty bottle of water with the cap tight and hold it here and there in a manner the you can observe the "rings" formed on the water surface. My mechanic mentor (in the early 70's) showed me this and it surprisingly helped isolate location of various vibrations (end to end, side to side)..
Ratchet straps over transmission and cinched tight to cross member can help eliminate mounts as source. put one over the bell housing to uni frame, and they can also be used to pull the front of engine down (up or side to side). Try different configurations.
With all that my gut feeling is it is internal, or t.c. Also not mentioned is fif there was any major structural replacement of the uni frame assy that may need inspected.