The bestest way in the whole world to fix it is of course prohibatively expensive. That would be to strip the car down to bare metal, remove the lead, (with lead exposure being what it is, grinding isn't recommended, but flame wicking might be) and then stich weld all seams, where just the factory tack weld existed before. This greatly increases chassis rigidity in and of itself. Then if you add braceing you have race car rigidity. Of course any rust anywhere will have been repaired during this process and use of a good quality steel will go a long way in helping resist future attacks on your cars structural integrity. Todays modern body fillers are not as prone to cracking and can also aid in protection of the understructure. But unless you are one of the rare artisans who can perform this work yourself, like Gary in Indiana, it will cost you more than the value of the car to have it done.
So yeah a brace kit is about the only way to slow it down. That are stop tracking the car and drive it under 40 mph for the rest of your life.
