McLarnin was never a lightweight champion. He lost a decision to Sammy Mandell in his challenge for that title. He should be judged as a welterweight.
I think you should reread my post again. I said he was labeled chinny and some people still remember him that way. He was even referred to as chinny in that awful Rocky Marciano biopic starring Jon Favreau. Then a good section of my post explores why I don't believe he had a durability problem (30 rounds with Armstrong, a win over Bummy Davis etc)
If Duran is having that much trouble against Viruet and De Jesus, I very much doubt that he would defeat Ambers. I think he'd get minced, Ambers was two tough and too strong. If he can lick Armstrong and Canzoneri, he can lick anybody.