I would talk to Kevin over at Southern Trails. He's got loads of saddles, trees, and different brands. I'm sure he can figure something that will work for you. Your horse might need a draft tree or a modified FQHB. Western Saddles, Trail Saddles, Barrel Saddle, Ranch Saddle Western Saddles, Trail Saddles, Wade Saddles, Ranch Saddles
If you can't find a treed saddle to work, then I'd HIGHLY recommend going treeless. The good ones are expensive, but they WILL work for horses like yours. I have some online friends with similar horses and they ride in either Sensation, Barefoot, or Black Forest saddles. Those seem to work best for wide, mutton withered horses. You'll want a good pad like a Skito saddle pad
with felt bottom and grip strips on the top. I'd use a wide 100% Mohair girth/cinch and an over-the-neck style breast collar. Both will aid in stability.
I am a heavyweight rider (over 250 lbs) and I ride in a treeless Bob Marshall Endurance (great saddles, but don't always work for super-wide horses). My horses do well even on long/hard rides (no sore backs, and always happy horses!). I use a Skito and a ThinLine saddle pad, as BMSS saddles don't have panels or any weight distribution system (like the others I recommended). My saddle is very secure, even during spooks or big hills. I have let total beginners ride in my saddle and they all really liked it! Treeless saddles are SO SO SOOOOO comfortable!! You won't ever want to go back to treed saddles
Going treeless is expensive though. If you can't find a used saddle (and they are still pricey), you'll pay out the nose for a new one. Then the saddle pad, special girth, and breastcollar will all set you back. My set up ended up costing me $1,500, and I bought my saddle USED! If you went with a Black Forest or Barefoot saddle (both cheaper, but well made) new or used, you might be able to keep it under $1,000 for your whole deal...