I assumed that he just misremembered that specific detail. Which, y'know, is still pretty silly, considering he bothered researching what a Model T looks like but not the timeline here. But mixing up pre-civil rights and pre-emancipation be much worse. That would completely undercut what's...
With that afterward, the manga makes a lot more. But you can see the seeds of Ryouta's subsequent manga being planted with how (relatively) moral and restrained the setup forces it to be. There must've been a nipple-shaped hole in the dam, and it burst.
The thing is, this world isn't that interesting. Nobody picked this manga up for the dad or the brother or the loser friend. If the story has played out, it would be best to wrap it up and start a new one. It might be sad to say goodbye to the characters, but dragging it out like modern...