The biggest change I'm aware of (and there are probably others, but I don't have any of my references here in front of me right now) are the vertical tail surfraces. The -6A vertical surfaces are much taller and slimmer than the tail surfaces on all the earlier versions. I also believe the horizantal tail surfaces were different. See if you can find a copy of Squadron's PBY in Action. That's about the cheapest reference I know of.
About 30 years ago I built a model of of a -6A by taking a Revell model of a 5A and grafting on the tail off or Revell's model of Jaques Cousteau's -6A which was very easy. Actually, if you could find a kit of Cousteau's model, all you would have to do is add the B-25 engine/nacelles and the doors in the belly to create a "Super CAt". (Well, you'd still have to sand off a lot of rivets)