Hi Colin,
Love following your threads, great pics.
Your carrier from my point of view is original, not a replica as it is built on the basis of a real carrier and will be made up of many original and refabricated parts.
Now if you were welding in the new sides you have just made onto the chassis of a 1979 Cortina 4.1 that would be a replica.
Even if it had a full set of your tool boxes on the rear!