There are custom length tubular control arms sold for your car that center the rear end in the wheel opening.
Just be careful that the upper control arms are not all plastic bushed as it will put your rear end in a bind with suspension travel. You need a set of bushings that has either a spherical steel bearing on one end (sitting in a nylon socket instead of a solid steel socket unless you are racing the car) or has rubber bushings on both ends because the upper rotates through two planes of motion simultaneously instead of just one like on the older cars that has a Panhard bar to center the rear in the middle of the car. Heim joints that have nylon bushings are rebuildable, last longer than a steel on steel connection and are a lot quieter. Control arms that include a Heim joint frequently include an adjustable top link to compensate for any change to the pinion angle caused by lowering the car.
You use new springs to control ride height (lowered below stock ride height) and to control ride quality with a set of shocks valved to the way you want to drive (I recommend buying adjustable shocks because my mood changes as frequently as track conditions change). If cost isn't an issue I would recommend using coil over shocks instead of the factory style coil springs and a separate shock. The control arms you choose has to match your suspension choice as a coil over mounts differently than the factory coil spring and shock.
You do not need tubular A-arms up front as there are no steering geometry issues with these cars. If you are going to use coil over shocks then you will have to buy tubular front control arms that have added camber gain an additional caster over stock for racing. These used to be found on circle track cars but they are not as common now.
Bigger brakes are nice but not required and you should already have twelve inch front disc brakes with eleven inch rear rotors and a parking/emergency brake system that works (unlike the earlier Cadillac rear disc brake).
I recommend lowering spindles (steering knuckles) over springs as it doesn't change the steering geometry and retains full suspension travel; but they can interfere with certain wheel and tire choices when you are not using stock size wheels.