Drum brakes are better than disc brakes. You don't need a power assist to hold your foot on the brake pedal as drum brakes are self activating. It takes a strong spring to pull them off the drum once they have stopped as the bind up.
The only problem with a drum brake is they are lighter than a disk brake. Sounds like a good thing with less un-sprung weight, but brakes work by friction converting mechanical energy into heat and with less mass they are not as good at absorbing heat as a big heavy rotor. If you look at a pick-up truck the disc brake caliper is basically the same, only the rotor is thicker by an eighth of an inch so there is more metal there to heat up. It is a disc brake's rotors ability to absorb a lot of heat as well as radiate that heat by way of a greater surface area that allows a disc brake car to stop repeatedly from a high speed without fading. After getting as thick as a truck brake, the Corvette started growing the rotor in diameter as well to include even more metal in the heat sink. So in brakes bigger is better.
The factory went to disc brakes not because they were better (they were originally sold with out power assist and did customers ever complain). They sold disc brakes on cars because they are cheaper to make and cheaper to install on the assembly line. The reason only the Corvette came with four wheel disc brakes for the longest time was because there has to be a tiny drum brake inside the rotor hub to allow for an emergency brake and a parking brake. Newer cars have a second mechanically operated caliper attached to the axle to act as an e-brake and parking brake, but because there is no power assist and they have to be constantly adjusted to account for wear they rarely ever work after the first 10,000 miles or so.
On my Camaros and Novas that I used to build as street cars that could compete in a drag race or a road race competitively I used the larger 11 inch rear drums in place of the factory supplied 9 inch drums. (GM rear axle flanges on cars all share the same bolt pattern so any car brake can be bolted on in place of any other). With the Impala the larger rear drums off of a pick-up truck won't work on an Impala because heavier trucks use a live axle (that big hub you see sticking out of the wheel on circle track race cars) and generally six or more wheel studs. So that trick won't work for you.
You can go wider if not larger in diameter so you can order the rear brakes off of a station wagon or cop car (that is why in parts books all parts have a disclaimer saying non-police or wagon because the parts are different on those vehicles). Also older cars before computer designed parts where built bigger because there was more of a safety factor built in. So old Buick brakes that had aluminum fined steel drums where wider than newer drums. But you have to buy the backing plates as well and I don't think they are reproduced.