Leonard beat a Hagler who had lost a step or two and imo , had awful corner advice. That's his resume at 160lbs, he isn't beating Carlos. Monzon never allowed anyone to dictate a fight,he would control this one with his jab and his body shots inside would take their toll on Ray's legs. Leonard had the heart to hang in there, but he would take a beating imo.
Monzon I think would stop Leonard. Leonard was not a good MW, the best he looked at MW was against the end stage of Hagler (a fight I will die on the hill that he lost) and he really didn't look phenomenal in that fight either except in bursts. If we take the best Leonard at MW (Hagler) against pretty much any Monzon through Napoles (even further), Monzon would grind Sugar down and stop him over 15. Probably would beat Leonard in less than 10. Sugar would have no hope of stopping him, he'd have no hope of winning the stamina war. He really lost that to an old Hagler (were the fight 15 rounds he would've been stopped). I just don't see Leonard's MW version beating Monzon.