The rear end assembly is the same. Meaning the third member, housing, axles, and brakes. Axle perches are in the correct location so it is a direct bolt-in.
The difference is the much longer leaf springs on the Cougar and the Iso clamp rubbers on the leaf springs.
A bit of trivia for you all.
The 5 lug 28 splined axles are all the same length (side for side) from 1967-1970 on all Cougar and Mustangs for both 8 inch and 9 inch rear ends.