i think that a rollcage would be the best option.
if the foam cracks you will never get it out and will likely have to replace that part of the frame.
i dont know the specifics of this frame but would adding steel in specific places be an option?
for older cars, you can increase the rigidity by boxing the frame.
your frame is (i gather) made of square/round tubing.
you could weld extra metal onto the frame in the places that are flexing.
it sounds like in your case the middle of the frame is the weakest link.
blindly strengthening the suspension parts would make the problem worse.
what you need to do is put extra metal into the middle section.
i believe that someone suggested a brace along the transmission tunnel.
IMO this is the way to go.
but hey, its your car. do what you want.