I have foam filled all voids in chassis at the same time I installed an AutoPower 4 point bolt in roll cage, and Extreme Dynomat from firewall to the taillights. The combination of these was a very noticeable difference in stiffness as well as ride quietness. The car does not have the normal drone, creaks or rattles. Rides nice and smooth. to give you an idea of stiffness, If I jack up the front right just after the front right tire leaves the ground the the rear right and left front come off the ground too (T tops in or out make no difference). I would recommend doing it. The foam is also so light that its not really much of a concern for adding weight.
