It would be easy for Ali to jab from the outside and keep Marciano off him. But Marciano, he was so tough and he would put alot of pressure on his opponents. But I don't think Ali would have a problem with him, I think he would box beautifully and win clearly by UD or TKO in the later rounds.
Ali by decision or a TKO on cuts, too fast for Marciano.
Marciano would cut really easy, so I can see that happening. I think Ali uses his size and will box his shoes off and win a WIDE UD and possibly a late TKO, like you said.