You might want to cut the inner part of the braces to gain some flex. You probably know about it anyway. That doesn't get you nearly as much flex as today's cars but still better than the ultra-stiff stock setup.
One day I'll have to experiment a bit more to try and get torsional flex out of the stock chassis. Even the composite version too stiff. Maybe mill down the center webbing, of lower the sides of the bathtub... or both... Maybe cutouts on the electronics side...don't know yet. One day...
I already use a center brace between the F and R gear cases so I'm not worried about longitudinal flex.