One thing that comes to mind is the torque converter.
If the engine was internally balanced, the torque converter should be too.
You may have gotten a converter that was meant for an externally balanced engine, but sold to you as an internally balanced one. (misboxed etc.)
The externally balanced one should have some big weights that look like 1/4'' x 1'' flat bar about an inch or so long welded to the side of it, (pretty easy to spot with the inspection cover off).
A neutrally balanced one should have no weights on it or very small ones.
It's pretty easy to knock them off with a good hammer and chisel.
Do you have a stock flex plate on it?
An aftermarket one might have weights on it or either of them could be bent.
I would contact your converter manufacturer to verify this information.............
