Problem with this alternate history is that the largest union corps (6th) was not engaged at Gettysburg and so would be a very strong rearguard. Lee was on a raid and being in Pennsylvania north of the army of the Potomac would have been a risk. Food and horse feed, yeah, but it has to be sent south for the raid to be effective. And ammo has to come north. See https://www.amazon.ca/How-North-Won-Military-History/dp/0252...