Well, I LOVE my Devoucoux Makila for my wide backed monster Shouldered Connemara Pony. He has moderate withers. They also have a new saddle out that I got to try--was really ideal for the horse you are describing.
I wanted a Schleese Wave--but they were not able to fit this pony in that model because he needed a completely different tree with swept back points.
My Wise Equestrian dressage saddle, with a gullet change system, works well on my wide backed, low withered mare. I am very long from hip to knee, as well. The flaps are cut a bit more forward, which helps when you have a long femur.
I have the world's longest femur. I didn't really notice this until the 5'10" girl at the barn said so. So I started lining mine up against peeps. Yup, my femur is akin to those 5'10" - 6' tall and I'm only 5'6".
I always had trouble with saddle "rubbing", now my guess is the two are related. I ride in a L'Apogee and LOVE it. Same saddle Silva Martin uses, btw. It comes in 2 "seats" at each size-regular and "short seat". I have the "short seat" which basically moves the sweet spot where you settle in. No rubs ever (well, that one time Janine Malone made me ride sitting/medium trot for 15 straight minutes at a fix-a-test but that's extreme). It's a monoflap so very close contact, and foam flocked, which I was really skeptical about until Kitty (saddle designer) talked to me about it.
My horse is only a medium wide, but considering he's only 15.2 and 1150 lbs, he's wide proportionately. He does not have "huge" shoulders, but he has a very handy front end and really uses those shoulders and goes really well in this saddle.