If you wish to seriously address the chassis flex in a Targa or a Cab it can be done, but not for the faint of heart. I do not think any bolt in quick fix will make a long term improvement. Think in term of the shear strength of the bolt as the most likely weak point.
Here was my solution for an open car. More that you may be looking for but flex very low and torsional rigidity is much improved.
The halo bar is tied into the front shock towers and the main hoop is tied into the rear shock towers.