Our '99 cobra developed a knock in the top end after the drivier beat on her a little bit doing donuts,(im not the driver) the car is mostly stock and has 90,000 miles on it. the knock is only noticeable around 3000 rpm or when shifting. the car was also hard to start afterwords. we are thinking about swapping 03/04 heads with mild cams. so the question is will this work and will the exhaust manifolds neeed to be changed due to larger exhaust ports? is this swap worth it or should we look into other options? any suggestions are appriciated, thanks.