>>149470132in The PTA Disbands the Springfield Brigade at Fort Springfield were clearly Union and attacked and killed greyshirts
The Color Yellow does unambiguously state that Virgil was a slave in Springfield - and this is implied to be the same Springfield - which pushes the town a lot further West than it should be given the founding date of 1796
but if it was founded after the Louisiana Purchase and Springfield/Shelbyville's expedition only set out in 1796 but founded the town inexplicably like 10 years later, they could be as far west as Montana or as far south as Louisiana, but unless Jebediah Springfield got SUPER lost and was incredibly old (given he was already an adult man with a career as a notorious pirate when he met George Washington), it couldn't be anywhere in the former Republic of Texas or Second Mexican Republic except maybe the two 1819 portions of the Spanish cession (and even that requires that it took 23 years for the colonists to found Springfield and Shelbyville)
Abe does say "I'll be in the cold, cold ground before I recognize Missouri" in Homer Badman, which may imply a Kansas or Oklahoma panhandle locale for Springfield, with Abe remembering the Missouri Compromise as a bad thing for his ancestors
but then they went to Canada anyway so it's possible that's an entirely different Springfield and (most) of the Springfield descendants of the other characters in that story simply come from very large family trees (Mr Burns graduated Yale in 1913, so that was possibly his father, or an uncle he knew; his mother appears to be vaguely southern based on her accent)