Using oem brakes requires either modifying the existing axle flange OR reflanging the rear using the old standard GM 4 bolt flange. Going back to 1964, there are have been 2 sizes of 4 bolt flanges. A small pattern used on most intermediate and smaller passenger cars and a large pattern used on fullsize cars and 1/2 ton fullsize trucks. The G bodies started using a unique flange in 1978 that is tall. It has 2 small 6 or 7mm screws at the bottom and uses the spring anchor to retain the top of the drum brake backing plate.
The 82-92 drum brake F bodies used the same flange. The disc brake F bodies used the older small 4 bolt flange. If you reflange the housing, then you could use oem stuff as long as it has a 4 3/4" stud bolt pattern. The Impala stuff wont work for 3 reasons:
The axle flange which the caliper bracket bolts to is the large pattern, the rotors are for a 5" stud pattern and the axle housing is slightly shorter to allow room for the parking brake assembly which mounts inside the rotor.
Best bet (and probably easiest) is to get a quality Baer racing set up. Jack Cotton is a dealer for them. Baer uses many OEM type pieces which allow you to purchase off the shelf pads and sometimes rotors; unlike Wilwood.