If Benn could wear him down, then so could JL. Also GM showed he didn't carry the same power after about 4 rounds.
I think mcClellan could outbox JLM, but he could never stop him ( unless by cuts ) nor even dent that chin.
I think McClellan would probably beat la Motta. The first thing I thought when I saw the matchup is that McClellan is a lot bigger than la Motta. And he is one of the best punchers ever who could box too. I might be in the minority but I think McClellan can do it.