Muhammad would beat Jack,by a competitive,yet clear,decision. Johnson's technique was excellent,for it's time,but he would n't beat a lot of fighters of eras later than his,let alone Ali.
No,but would Johnson last 15 against Ali in HIS era ? Anyway,if trained for 20 odds rounds,Ali could do it.
Maybe, I don't know. Ali was bigger and weighed a bit more. But I don't think much effective punching would be inside anyway, Ali would want to fight at a distance
I wouldn't give Johnson a prayer in this match-up. He would probably make my top ten ATG list, but in h2h terms (admittedly hypothetically), he doesn't crack the top 20. I would probably pick Norton over Jack as well.