I posted back to you on this topic on another thread James, but here it is again, the outers are most certainly different. Inners could be the same, I am not sure. But outers, I am very sure. The main difference is by the rockers, the Cougar ones have accomodation for the rocker molding clip stud, the mustang ones have no such accomodation and if you have or plan to use rocker moldings, you will have a problem with the rear clips because of it.
Time for an update post. The inner and outer drivers side I tried were Dynacorn parts (from CJ pony parts), they are nice pieces but not even close to the Cougar shape. The inner is ~1 inch narrower and the arc is not big enough so you would have to splice in a vertical piece in the middle. Also the curve at the front is not the same.
The outer is even more challenging as it's curve is also not big enough and it does not follow the quarter panel wheel lip profile so it would have to be cut up to make that work. I didn't spend too much time on this, I would have been willing to make serious mods to the outer if the inner worked but considering all the work required just for the inner I think I would rather spend the time patching my existing wheel houses.
One would think there would be a good market for reproduction Cougar wheel houses, I'd happily pay double or triple what the Mustang pieces go for.
Good luck everybody
Best thing to do is purchase a set from a cougar vendor, mainly WCCC .They`ll cut out what you need and all you will have to do is break out the drill for countersink holes and spot weld cutter and remove the excess metal and fit it to your application. I too tried using mustang inner and outer wheelhouses and they don't even come close. A waste of time and money IMHO. Go with cougar parts I know cutting up a good car is hard to do but its the only way to get the right fit for the quarter panel.
Agreed, Don Rush at West Coast Classic is going to send me some grade B wheel houses. I can get grade A but they would come with the trunk floors for an additional $400 but I have already purchased new trunk floors from John's Classic Cougars.