vibration issue

I have a vibration issue that I will need to use the process of elimination to diagnose. I thought that this issue would have fixed itself after my most recent rebuild of this car. A little history: I acquired a 1970 340 TA motor back in 1980. It was in parts, but complete. I had it balanced and built and installed in my 74' Duster. It was backed to a 904 Tranni. This is where my memory is a little sketchy. I can't recall a vibration problem with the 904, but it didn't last very long with the 340, so I found a 727, bought a TCI coverter, installed a 8 3/4 rear and put in 410 gears. I recall vibration issues at around 2800 to 3000 rpm after that and remember changing to 323 gears to make things more streetable and it also reduced the vibration at highway speeds. Fast forward to my most recent build of the car. The engine was disassembled and balanced again. I added a GV overdrive and went back to 410's. Also urethane engine mounts, a new Summit balancer (internal balance). I still have a vibration that is very annoying and makes the car uncomfortable to drive. I am getting ready to drop the tranni and switch out the TCI converter with a new Hughes. I have read some previous forum questions regarding this issue. One specifically mentioned some of the early aftermarket converters causing vibration issues. Before I do this, I wanted to reach out for any other thoughts regarding this issue. Also wondered if anyone thought I should replace the flexplate while I have the tranni out. Doubting that the flexplate could be the issue.