I would look for an "Arabian tree" or FQHB.
I've found, with my Arab, that most SQHB don't leave enough room for how flat through the back she is (and your Arab seems to be built similarly to mine).
The saddles that have been working great for me are an Australian saddle in wide (thought I wouldn't like it, but I've found that it's so much MORE secure than a western saddle! And since I trail ride, security is important) or a Bighorn cordura saddle with FQHB. If you can, I'd really look into an Aussie saddle, I can't even tell you how much I love mine. And it fits my mare so well, I don't even know. Haha!
Here are links to both:
(I got the "Synthetic Endurance", the 3rd one down, from the clearance section. It's just as nice, just because of one scrape, they marked it down $30!) Kimberley Series Aussie Saddles
Here's the Bighorn: 15" 16" Big Horn Cordura Saddle FQHB BROWN or BLACK 206 250 205 251