Maybe narrowing the saddle fitting down to your horse being a Peruvian Paso in conformation might help. I am very familiar with a Peruvian Paso mare owned by a friend of mine. Brisa, the PP I know has a rather thick body conformation. Gosh, I wish I could be of more help.
Brisa, the PP I know is a sweetheart, but a handful for her senior aged owner. Brisa's owner has hired a younger girl to ride Brisa on an every other day schedule. The hired rider takes Brisa on hours long rides and still can not wear or tire her out. She goes out pounding and comes back pounding the gait.