You may want to start with just rebuilding/checking the stock suspension and make sure everything is correctly aligned/adjusted. The handling of these cars may not be impressive by today’s standard, but I feel that many times, the complaints come from parts that are worn, alignment that is off, etc. No big sway bar in the world is going to help if the tie rods are loose or if the strut rod bushings are dry rotted
From there, there are many modifications you can make: Shelby upper A-arm drop, bigger front sway bar, GT springs, good shocks. Some of these changes are easiest to install when you have everything apart anyway while others can easily be added later.