Good advice given with 13 cabs.
It is not just a simple case of 'cab back and roof' as you now probably know, as you really do need to graft a complete cab floor and framework on to make it right. As already mentioned you have at least then salvaged the real thing for future use, or perhaps someone with an HUW they want to rebuild?
Another difference is that the front wings on a C8A are narrower than their 15cwt counterparts, so a serviceable/repairable 13 cab is definately the way to go. You do at least have a good nose section, doors, seats, engine cover and dash assembly.
As I write this however I wonder if the task of restoring to its correct HUW configuration would in fact be easier !!?
Ian