I wondered how your Gila River stop would be on the route of a stage from St. Louis to San Francisco. But the article explains they took a long curving route through Texas and the southwest. The trail even fords the Rio Grande at one point. Good stuff, if I do say so myself.
Late response.
I would guess is needed to be south to avoid the high mtns and snow. Probably a route laying a foundation I-8 to I-10 after the Mohave thru the Sonoran desert. Water would be the issue.