I've got a couple of Simcos with full quarter horse bars that are wide. I ride my fairly wide Rocky Mountain in a Circle Y Park and Trail with an arab tree. Didn't know it had an arab tree when I bought it but it fit well because it is shorter. A gaited saddle may be way to go for clearance at the shoulders.
OP-I'm having a similar problem with Willow. She has a short back and doesn't have prominent withers, but she is wide chested. I need around a 27 inch skirt with about a 6 inch gullet.
I've found most FQH are too wide causing them to sit low pinching the withers and Semi's are too narrow.
The only saddle that was a close fit was a Royal King gaited saddle my sister had. From what I've researched an Arab tree should have a wider angle but not as wide as a FQH. I haven't tried one out on her yet though.