74 Dart Vibration

In need of help. I know there is a tone of threads on here for a vibration in a 360-727 set up. I have read threw a lot of them check a bunch of stuff and am still failing to find the issue.
So as for what I have done to the car. I updated the drive train to rebuilt 360 with a rebuilt 727 backing it. Motor is supposed to be balanced to factory specs witch call for all the external parts to balance it out.TCI neutral balanced Saturday night special converter, B&M flex plate for the external balanced 360, new balancer for the 360, and all new mounts for the motor and trans. That is how the car is currently set up.
Now for what has been done I've changed out the B&M converter for the TCI because the center hub on the B&M was bottoming out in the crank.
I've checked the flex plate with another brand new one no difference. I've changed out the tranny mount. I changed out the original balancer for brand new aftermarket stock replacement due to a grove where the seal rides.
I've tried running the car with no belts on it.
But this car still has a pretty good vibration at 1500-1800 RPM and it will smooth out as the RPM picks up but never goes away. Doesn't matter if its in park or gear vibration is the same.
Nothing that has been done this far has changed the vibration one bit.
Any and all help is very much appreciated I'm getting tired of banging my head on the wall.