Here is what I did on a 1958 Karmann Ghia. This triangulated the frame horns and tied them into the torsion tube. This work out very well. I’m sure you could do something similar and tie it in to the rear subframe. If you try this, put the transmission rear mount on before you weld on the frame horns, as the heat will move them! I also used this to tie into a roll ball, that is the upper plates in the picture.