I don't think reinforcing the connecting bracket will resolve the problem. The rear cradle is what twists. There are only four points, the two rear bobbins and then the brackets bolted to the bulkhead, that resist the torsional twisting forces of the springs and shocks. Think of a shoe box and how much stiffer it becomes with the lid on. The 300 Sport has a massive bracket linking the rear bobbins to the front of the cradle near the bulkhead. The 300 Sport had a lighter, hand laid body, not as thick or strong as the varimold bodies, so Lotus had to restore the strength somehow.
What I did several years ago was two things. Firstly I eliminated the bobbins and made alloy plates. The bobbins work in tension and there's not much surface area so eventually they bust out. With upper and lower plates about 50mm square and 6mm thick and several years of use, I've had no more problems.
The second thing I did was making additional brackets halfway between the bobbins and the bulkhead. I used square tubing about 400mm long bolted into the inverted "u" channels on each side of the inner body molds. You can feel those channels by running your fingers under the air cleaner and corresponding side next to the exhaust manifold. From the mid-point of the square tubing I welded triangulated brackets that bolt to the engine mount plates on the chassis. That part of the body mold is very strong, being a box to hold the petrol tanks and inner guards.
I well remember my delight the first time I drove my car over a rough road how much stiffer it was with the new brackets. A bit of effort but worth it. Only weigh a kilo or two, well hidden and can't be seen without a very close look.