I got a 9 inch rear end out of a 65 Thunderbird and its slightly wider than the Cougar 8 inch rear end. My question is "how does a wider rear end affect handling"? I'm talking an extra inch on each side but NOT poking out the wheel well.
I can't imagine why it would cause you to have more (or less) oversteer or understeer. Those issues are related to a difference in traction between the front and rear tires while turning. Understeer - front tires plow forward due to reduced traction. Oversteer - rear tires slide out due to reduced traction. A wider rear end, as Al implied, could mean you would need more ballast weight on the passenger side.