[1QuickS]

The angle will fail by buckling the leg pointing toward the floor in compression with very little load application; it would be better if rotated 90 degrees and mounted using spacers to clear the leg from the chassis...better yet to use a rectangular tube. However, be aware that the mounting points were not designed to be used for torsional chassis stiffness measurements and as such may be limited to what load they can take. Your long lever arm will apply quite a force to the bolts in tension and compression and they were designed more for lateral and compressive loading; not for loading in tension.

I like the digital scale; simple to measure load applied to the lever arm.

Use two digital levels and take the difference in level readings between them to calculate chassis twist.

I expect the plot will be non-linear until you generate enough twist to get the entire structure working as a unit.

It would also be good to know what stiffness you might want to achieve. This can be deduced by estimating the ultimate grip at a front tire and then use this as a load condition for which you want to control chassis twist to some acceptable level such as 1 degree. These two numbers (tire grip and torsional wind-up) will provide a target for chassis stiffness. The distance from the road to the torsional centroid axis of the chassis could be assumed to be about 24" unless a better number is known. This is my guess for a coupe but the centroid height for a Targa would be more like 12". Certainly there are those with greater technical contribution for this but I'm just thinking the process through for your awareness.

I expect the 935 cross brace was to help stiffen top of shock tower by providing another load path to a support point. Also, by cross-bracing the open front compartment it becomes more rigid as a unit and helps reduce deflections at the suspension pick-ups.

It would be VERY interesting to have a coupe to compare your torsional stiffness numbers to. At least there would be a standard to aim for.