The templates are great to give you an idea of what will fit your horse, but taking him to a saddle shop and trying on lots of saddles, sitting in lots of saddles, is really important.
Someone somewhere said that fitting an English saddle is easier, it really isn’t. You still have to have a wide enough channel vs gullet, the angle of the shoulders is so important. If you have a horse with round shoulders, both english and western can be tough to fit.
Start with the template. Follow instructions to figure out how to do a template of your horses back, especially right behind the shoulder. The best way is using a flexible curve that you can buy at a hobby store or off of Amazon. Works much better than a coat hanger or wire. Same way english fitters do. Then put that tracing on card stock or a file folder and compare it to different saddles for the correct angle.
Do you have pictures of this horse? What type of saddle fits him with his prior owners?
Good luck and we want to see pictures.
Or and for what its worth, I have had both an endurance saddle and an Aussie. Nothing is more comfortable than the right western saddle for long rides. Not even my extremely comfortable dressage saddle. Especially on hills etc. But sit in those too.