If the tree is too narrow, no matter what you do, the back of the horse will be sore. Be sure you have enough clearance between the horse's withers and the gullet of the saddle. Double-rigged saddles have some advantages on mountain trails. They should be rigged so the cinch is a little farther back to prevent sores behind the front leg, the strap between the cinches should be adjusted and the back cinch in contact with the horse's stomach to prevent sideways movement and to help keep the pads in place. Three quarter rigging is the best position for mountain riding.
Some people prefer a saddle with a high cantle for mountain use.
However, the higher cantle shouldn't be used to brace against when riding uphill, keep your weight forward and help your horse. Stirrups should be adjusted so that you can place just three fingers between your crotch and the saddle when standing in the stirrups. A properly adjusted breast collar is an advantage when riding in rugged country. Breast collars should be fastened to the rings installed for that purpose or off the 'D' rings, and in a manner so there is no possibility of interference with the horse's breathing. A crupper or britchin can also be added and is recommended for a horse with poor withers or round back.