I'm not familiar with 8.5 cert ( it may cover all of this + some) but an; anti roll bar, x brace under the car, x brace in the doors and support bars from the motor plate to the snout bars will keep it from twisting.
A lot of people overlook the motor plate bolts too. 2 bolts will allow the frame to pivot. 4 bolts (2 each side) will help stiffing it up a good bit.