Pre-exile Ali would've run circles around Joe Louis. That's not meant as an insult to Louis, but the young Ali had the hand/foot speed of a welterweight and the body of a heavyweight. Louis would've been chasing him all night, getting battered with jabs and quick hooks in the process.
Post-exile Ali couldn't avoid punches the same way, so that fight would be much more a tossup. I still think post-exile Ali beats Louis 7 out of 10, but not 10 out of 10 like the young Ali.