If you want to stiffen the chassis, seem weld it, rear anti roll bar, thicker anti roll bars combine this with a set of coilovers, poly bushes on everything, adjustable top mounts and a proper geometry set up and your car will handle like a train.
I wouldnt bother with a roll cage unless it's a die hard track toy. Even in my track car I refused to put a cage in because it weighed too much!