I would go with Monzon by decision. Both men could really take a punch, I think Monzon hit harder, had a better jab.
You must be new. On this forum, LaMotta could beat King Kong with one hand and Godzilla with the other ... at the same time. He was practically unbeatable. Amazing he ever lost a fight.
Monzon was just a higher class fighter while being just as tough, a granite beard, knew distance and pace as well as anyone. He would win and win by a significant margin.
It would have been another night at the office for Monzon, who was was stronger than Robinson and hence wouldn't have been bothered as much by LaMotta. He would have had his way with Jake all night long...fighting somewhat similarly as he did with Briscoe in fight ll.