Mike, you sound just like I did 15 years ago when I made my first pack saddle. Back in them days I more time than money so I decided to try and make my own pack saddle.
I had an older set of deckers that had been packed with for years. They have some flair, rock and twist just like a ridding saddle, so I used them as a pattern ( so to speak). I found some cottonwood down by the river, used a chain saw ruff cut them and sanded them to shape. Two weeks latter they cracked and split all over. So i changed to spruce and carved and sanded them to shape. After I had them done I made my arches out of round rod and welded on my feet. I sure was proud, until I placed them on my horse. They had gaps and looked real bad.
I asked my dad which has been packing a long time to tell me what was wrong with my tree. He said the tree that i had used as a pattern was from a "special" horse and he had rasped that tree to fit that horse. That is or was very common in those days to rasp on a flat board ( the tree bars) to fit a horse or mule. It was also common to heat the arches to bend them fit better as well. I think pack saddle makers just took less time to make the trees. I would start with a decker , because you can change the bar angles after it made, which you can't with wood. When I make pack saddle now I still use metal arches just in case i need to adjust them a little after the bars are made. Like another author stated wider bars are better and i would agree with that. Just make some up and stick them on your horse or mule until they fit good. Good luck.