That’s a great idea! The only thing is that places the port of Guaymas in American hands, which I dont see the Americans needing given that they now have practically every port on the Pacific Coast. So what I'm thinking is the border goes like so:
Perhaps instead of a straight line, the border could follow the course of a river further south in OTL Mexico? The US seemed keen on using rivers as borders in OTL Treaty of Guadeloupe Hidalgo, and the only reason the Arizona and New Mexico borders are straight is due to a need for railroad land in the future. Looking at this map, the Conchos and Maya rivers form a natural border in the West, only needing connecting with a short line (as in OTL occurred between the Gila and Rio Grande). If you want the border further North, you could use the Yaqui river as well. Just a suggestion, of course! I'm enjoying the timeline so far.