Yes, longer drive shafts due to the longer wheel base, between Cougar and Mustang.
For 1967, 8 inch and 9 inch rear axles are the same between Cougar and Mustang. Cougar are all 5 lug though, no 6 cylinder engines in Cougars. Some rear (and front) suspension components are different on a Cougar, but the rear axles assemblies are the same.