Here is something I didn't really understand about horses and saddles, and learned from trying to get something to fit Trooper. Others can jump in and say if I'm wrong...
A number of English saddles now come with an interchangeable gullet. The gullet is the inverted V where the saddle fits on the 'shoulder', which is really behind the shoulder. The interchangeable gullets make that upside down V change angle from narrow to wide. This is supposed to make it possible for the saddle to fit any horse.
However, take that inverted V and cut the top off, and bridge the gap with a horizontal line. That is the gullet width. Just as horses have different angles where the saddle meets the horse, they also have different widths. Some quarter horses (I'm told, I don't own any) would need a wide gap with a narrow angle. Trooper, OTOH, needs a narrow gap (width) with a wide angle.
But the gullets all have the same width. All you can adjust is the angle.
If the width is too small, the saddle will perch up high in front even if the angle is perfect. If the width is too large, the saddle will settle and be low on the withers, even if the angle is perfect.
In the first case, I think you are screwed. I also think most manufacturers build a wide gap for Quarter horses, because they know that a pad can fill in the too wide gap for those of us with narrower horses.
If the horse has very long withers (Trooper), screwing up the gap will cause the saddle to hit against his withers where the withers end. You can have enough fingers of clearance on the front of the saddle, but still rub his withers at their rear. And some saddles will NEVER have enough clearance at the rear of the withers because they are shaped wrong. A forgiving horse, like Trooper, will endure the contact, but he may lack enthusiasm for trotting or sometimes flip his ears back or raise his head and hollow his back. He tries, but sometimes it will bug the heck out of him.
Ideally, we would all ride in custom saddles, or be able to take our horse to a tack shop and get good advice. Real world, the tack shops within 150 miles of me are staffed by morons who will try to sell you any saddle they have in stock. The only shop I know of within 200 miles with English saddles sell Collegiate saddles. I'm not slamming them, but like my Bates, the interchangeable gullets change angles, not widths.
And my family and I ride western or English, depending on the day, the horse, the goal, etc. We live with compromises in saddles fitting us, and our two horses have small but significant differences in their backs.
With a western saddle, what works well with Trooper is a saddle pad with a long cutout in front. It doesn't do much good to have a curve, because the curved top will leave the pad hitting his hips. A long split allows for some extra clearance on his withers. Even so, I may be stuck with using saddles that have a sharper rise from back to front than I like. I'm not sure it is possible to put the flat type of saddle I like on Trooper, because it would sit so far back.
With an English saddle (and Australian style), a wither pad fills in the gap. My mare doesn't need it, but Trooper does (Mia needs a slightly wider gap than Trooper, and has shorter withers). It is a compromise, but since he is ridden perhaps 4-5 hours/week (and usually with the western saddle...maybe 1 hour/week English), it works. The alternative would be a custom built saddle, but a custom built English saddle used by me alone for an hour or two a week tops is WAY out of budget.
It is also outside necessity. I think Trooper is just happy to have owners who try to make the saddle work, and who don't use spurs...he has scars on each side where someone spurred 2" diameter holes through his skin.
If I needed to compete with Trooper, or needed to work him for 12 hours days, my saddle needs would be different. I sometimes jog in cheap sneakers, and have done so for 40 years. I don't go that fast or far, and I get away with it. If I ran competitively, I would need top of the line shoes. When I peaked out running 30+ miles/week, I needed very good shoes. But I now do 15-20 miles/week, so I can compromise.
Sorry for the novel. Saddle fit is an important part of caring for a horse. I'm learning, and still have far to go.