Side to side inside capping: 56 7/8"
Back of rear bulkhead to rear: 43 3/4"
Tailgate width: 35 7/8"
Height from capping to tailgate bottom: 20"
Also of note, the capping on the rear bulkhead iextends 7/8" beyond the cab.
The cappings were mixed and matched and welded to do the job, the kit did not include them. In UK, thay may be galvanized, but I've never seen a double cab in person except the one I built. This is one of those projects where you do a lot of test fittings until you get it right. The rear bulkhead needs to be under the cab, so it's location is dependent on the cab. Rear door fit (i.e. gap) also depends on the cab top and sides. I will post additional photos if you tell me what you want.
In back, I use a NAS style swing tailgate and spare tire carrier.