Things are a little slow on the forum, so here are a few progress pictures of my gr4 project.
I decided to add a upper rigidity-bar and hope that it will comply with FIA rules. I've already added some smaller stiffening plates in the rear part of the chassis (see earlier in the thread), but this bar, in my view, is the most important bolt-on addition you can add to the chassis, basically preventing the shock towers from flexing under load and hard braking. In combination with the roll cage this should stiffen the rear up considerebly.
This is a piece from Hall.