As I understand it 1968 used Girling Corvette Brakes (the rear rotors withg a Corvette four piston Girling brake caliper. In 1969 GM introduced their own single piston full floating caliper (the Girling used a fixed bridge where only the pistons moved). Because the engineers where concerned about dragging down a heavy B-body at the new Interstate speeds that were just opening to public use in the late sixties Chevrolet hedged their bets and used a two year only dedicated single piston caliper just for the B-body (it was unique).
This special brake was used on 1969-'70 model years but was backward compatible to 1965. The 1968 Corvette style brakes can also be installed on all B-body cars from 1965-'70 if you can find the front wheel bearing hubs that are the only Impala part; because everything else is available from Corvette restoration vendors at Corvette prices.
In 1971 all GM cars and light trucks used the same disc brake caliper that was first offered on Chevelles, Camaros, and Nova cars in 1969. So all brake parts interchange with only the steering knuckle being unique to the car model.