Tough one, real tough. I was a fan of both, and won several thousand dollars when Nunn shelled Frank Tate. I would take Rocky Valdez here, but not by a whole lot. He would have to come on late to win as Nunn when he was on top of it really knew how to use his height and reach to full advantage, but he was no Carlos Monzon, and that's what it would have taken to beat Rocky. Valdez TKO late.
I think Nunn might be able to slap flurry and hug his way to an ugly, possibly controversial decision. Though I wouldn't rule out Valdez pulling the fight out late with a Toney-esque KO.
You wouldn't concur with Sly much, trust me. You guys are polar opposites. He generally felt that old time fighters were crap, and he acted like Floyd created the sport. I assure you, he never watched Valdez a day in his life.