If he had Eddie Futch in his corner (as he did for much of his career) he would put up a good fight. With Futch's advice and McCullough's heart and determination, he could make for an exciting fight for awhile. Sorta like Pazienza did vs Roy Jones, but even better than Paz because he would be fighting guys at his (and their) natural weight. But, also like Paz, he would eventually find himself terribly outgunned. I see him getting TKO'd vs. Zarate, Olivares and Jofre.
Would he be competitive? Sure he would, why not? Would he win many against these guys? No, i dont think so. Would be entertaining though
He'd take some of the most horrifying beatings you've ever seen, but would get his licks in. Wouldn't beat anyone that good though.