I would pull the inspection plate and have a look. Bent flex plate, missing a bolt???
Both Torque Converters that I have installed on my car have had balancing weights*(simular to a car wheel, but welded on), I don't know if this is common or normal, but you might see if you have them or if it looks like something is missing.

* one was original to the car, the other was a New replacement.

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1964 Dart GT

Those problems would vary with RPM - I had a '64 Dart that someone swapped later engine or crankshaft without using the bushing. Basically the same as an unbalanced torque converter. Anyway, at idle it had a bad vibration that would smooth out at higher rpms. but then the vibration came back even worse at high speeds, like 70 mph or faster.

Josh, you were correct, the source of my low-idle vibration was indeed the hard poly mounts. Doc confirmed it as well and advised me to switch them. Swapped the original rubber mounts back in and the vibration completely went away, now the car runs silent inside.

I still have a poly transmission mount - hear a little noise that might be coming from this mount. I may replace it with a traditional rubber mount if it starts bugging me but I'm enjoying the newfound silence at idle too much right now to care.

Wish I hadn't sprung for those poly mounts with my rebuild after all. They came so highly-rated. Had no idea they'd transfer so much vibration inside. A word of caution to anyone who plans to use them for their street vehicle and likes the quiet stock ride of the slant. I wouldn't use these.

If you would like to get rid of them let me know. I wish that I had used them instead of the new rubbers in my W-100 but at the time did not bother to take the time to take my time and ask the questions I should have asked.

P.M no good but E-mail is jhason2@yahoo.com