engine/trans vibration

Is the TC centered properly in the crank? You know about the 65 and older TC to the 67 and later crank-pilot mismatch right?

Is it there in every gear?

Did you change the trans mount during the other changes?
I am not sure what you mean by "centered properly", but this converter was built about 7 years ago to match the magnum eng to the 65 904 because I found the original owner who installed the eng/trans had the wrong convertor, I purchased the car unfinished...., when i go it on the road i noticed an issue and had the trans rebuilt and new converter made back then 7 years ago, no issue with that new converter since, during this recent rebuild the trans shop said the convertor was a "a real nice unit" and only did a "inspection" and "freshing up" of the converter.
The cyclical vibration is mostly prevalent when the trans is in drive: 3rd gear