I'm no good at conformation, but I think that you are not going to have a fun time finding a saddle to fit that horse.
As someone who went through several poorly-fitting saddles, over a year of headaches, and a lamed pony, I have to say that if I ever buy a riding horse again I will buy one that has a back that is much closer to the norm. I do NOT want to go through that again.
"Saddle fit -- it's a no brainer!"" - random person