The engine and trans mounts would work, you'll need a C6 with a Lima bellhousing since that is what a 400 has. The shock towers will get tight but may fit.
Why not just put a Lima in it (like Bob, rktmn)? Yes, you can build a 400 up (although very few do) but a Lima would give you gobs of torque and would not require much to be a rocket in a Cougar. Don't even think about a 400 in stock form, they are smogged out anemic engines. Just a thought.