Most cougars had 7.5 rears. The 88 xr7 had an 8.8....either of them will be wider than the fox rear. Supposedly, longer axles and offset drums.
The rear will bolt up but may push the wheels out in the rear about 1/2 inch per side. If that cougar has an 8.8 it may have been swapped from another vehicle.
Disclaimer...this info was found on Google search for cougar rear end width.