My wide and flat backed little lady has had saddle fit issues for ages, I got a different (and highly recommended) local saddler to come and take a look who has said a hoop tree saddle will be the only type to fit her properly. I hadn't heard of them before so it was a bit of a discovery for me! She explained that if your horse is of the shape that needs a hoop tree, that a normal v shaped tree isn't likely to ever fit, as no matter how wide you make it there will always be certain points digging in because the saddle doesn't follow the curvature of the horse. It seems to be answering an awful lot of questions that others weren't able to.
It could be worth consulting a local saddler if you have one to discuss your options? It may save you paying out on getting one that may not fit in the long run?