You could always get a K-member brace (g-load). If that doesn't work....looks like you should add some monkey bars to the interior (roll bar). I suggest Maximum Motorsports for your bracing needs.
Where does it feel like the chassis is twisting from and what kind of driving are you doing when it is twisting?
Answer these questions and I think that will better direct your answers. Verts have never been known to be a rigid car so a roll bar might be the answer even though you do not want one.