Charts are showing an 18" seat on Western Saddles for the specs you posted.
One reason 17" Seat are the maximum size as standard is because of the length of the bar in the Saddle Tree, if the bar is long enough to accommodate an 18" seat, care must be used in selecting a Horse with a long enough back, too long a bar on a Horse and too much pressure is put on ribs that are not designed for such weights/pressures.
Going custom means someone will be talking to a semi or custom saddle maker, this insure both the rider and the horse will be happy.
I would suggest a Saddle with an A-fork or Wade type Swell as the narrow fronts offer a touch more thigh room also.
May all your Trails be happy and safe ones