Drivetrain Vibration

Hey folks!

I have really strong vibrations at 2800 rpm and up.
In part throttle the steeringwheel is even shaking.
It is in neutral the same as driving.
Driving over 60 is real bad.
But sometimes even under 60 partthrottle is very bad and all starts shaking.

I swaped the rearend and I did the whole engine but still there.
I changed the engine mounts and installed a TCI Rattler but still bad.
I tried without fan, powersteering pump, alternator aaaand it's still bad.

I don't think it't the engine because I totally changed it (heads, cam, lifter, piston,...) and it's still like before.
Changing the ignition curves (MSD E-Curve) was also no big help.

The vibration comes through the floor I guess because sometimes I feel like I got massage seats in my Duster...

I had to adjust the flexplate a litlle because it didn't fit but I think it's no big deal..
Maybe It's the converter but I don't know what could be wrong with that.
It's ballanced and for internal ballanced engines like my 318.

I don't know what to do anymore..

Thanks anyway.
I did a swap from a 904 to 727 years ago and had a terrible vibration as well both in neutral and driving. Found the transmission I had had a casting on the extension housing that made it touch the trans tunnel after installation. Once I resolved that, the vibration was gone.