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Advantages are that a two inch drop spindle lowers the car without changing the factory steering geometry or affecting the suspension travel. The disadvantage is it limits how wide a wheel you can run before the rim contacts the steering knuckle. If you increase the wheel diameter the entire steering knuckle will fit inside the wheel, but you are not going to be able to run the stock fourteen inch wheel covers if you do. Which is an important consideration if you have an SS model which was only a collection of chrome trim and a pair of bucket seats (the wheel covers being a means of identifying your car as an SS at a distance).
I would not run four wheel disc brakes unless you are thinking of road racing the car. Eleven by two inch rear drum brakes are more than adequate to safely stop the car on modern interstate roads (assuming you can actually find a cleared section of road bed that allows you to actually get up to the 70 mph speed limit).
Disc brakes were used by the factory on the front because they were cheaper to make and install on the car not because they are better at stopping the car. Disc brakes are used on race cars, rail trains, and aircraft because they can be heated up to a high temperature, and then quickly shed the heat. It is the ability to shed heat that makes a disc brake more useful than a drum brake to racers, not because it works better (they don't; which is why big semi trucks still rely upon drum brakes to stop over weight trucks on the interstate).
Disc brakes were not used on the back because you still have to have a drum brake mechanism to get the parking/emergency brake to work. It doubled the production costs of the rear brakes on a car with rear disc brakes which is why they were only used on vehicles with a lot of built in profit margin such as the Corvette (which actually needs four wheel discs if driven hard) and the Cadillac which had them only as a marketing ploy.
Drum brakes are self actuating which means it takes very little pedal effort to apply the brakes (most of the work is in overcoming the powerful springs that retract the shoes). Disc brakes requires a lot higher line pressure to clamp the pads to the rotor, which results in more effort from you and your right leg. This is why almost all cars equipped with disc brakes are power assist. Most can generate enough pressure to stop the car but your leg will tire in trying to hold the car from creeping forward at a light without a powered assist.