I think the Duran that fought SRL was in shape and ready to roll. He just couldn't catch a very negative but effective SRL and nor would the guy who came in against Barkley. Speed and movement were his kryptonite at the higher weights and ages, certainly post SRL 1 just to cover both theories on that one off. McCallum would have been right in front of him. He was also quite slow but had exceptional timing. Watson is about the only decent guy he stopped at 160 and the clinic he put on against him would not be occurring even against this aged Duran. If he felt the need to he would have slipped back a gear like he did against Hagler and spoiled somewhat. That's my take. You are right that he was old and had only so much. I still think he would have won some rounds tho - could be wrong. McCallum was fighting at a consistently high level against very good opposition. I don't however see him overpowering someone of Duran's ilk even then.
Duran in the same shape and mindset as the Barkley fight would give it a go but Mccallum wouldn't be stupid enough to brawl with.Duran like the blade did.Duran lost a decision to Simms who wasn't as good as Mike .Middleweight wasn't the best for Duran plenty of fighters in the top ten back then would of beaten him .points decision for the body snatcher in a rather disappointing contest.
Mc Callum on points. Duran would be more competitive than against Leaonard III. A stoppage win is out of question.
I don't think it has to do with in shape, I think some styles are better for Duran and speed was always his achilles. I think Mike would be an ok style for Duran, but Mike is also qucker than Duran at this point... i think he outpoints Duran in a good fight.