Hi all,
interesting that someone solved this problem :-)
I had to change the silentblocks after the vibrations were already unpleasant with the repair and the stiffened frame
Here are a couple of photos of the frame rebuild.
Steel pipes, four pieces are pressed into the flood frame with a big hammer, as much as possible.
After sawing off the blanks of the frame, you will see the extremely thin material of the frame and the hole into which the reinforcement will be inserted.
The pipe is not painted, it is welded through the hole in several places. It gradually became firmly connected to the original frame.
The rib is simply welded.
There are only two Silentblocks, in the front, simply cut off and fixed with two plates.

https://drive.google.com/file/d/1FeMv1Rv469X8lOMuqRrudxJAOwweHn_a/view?usp=share_link
https://drive.google.com/file/d/1FeMv1Rv469X8lOMuqRrudxJAOwweHn_a/view?usp=share_link
https://drive.google.com/file/d/1FmdOBrw6INnfWBUfk5Zy42JJsUlSXS-S/view?usp=share_link
https://drive.google.com/file/d/1FstOeDW5_NbPHbTe71fOJJ75mOR-AAUG/view?usp=share_link
https://drive.google.com/file/d/1Fw6Qs9Ky_aH2jC0vqzVOHcPrmWkE3sRZ/view?usp=share_link
Result is a totally changed m3w, rigid, solid with super driving characteristics and no vibrations :-)

Andrej