It maybe possible that he smothers McCallum with workrate. He has skills too. McCallum certainly isn't stopping him.
Gavilan never had to deal with a body attack like McCallum`s, his smaller body would gas after that assault.
Ever seen SRR's body attack? Gavilan went 25 rounds with him without caving. The guy went over 140 fights without being stopped and has plenty of names on that resume of his. He fought plenty of middleweights including Olsen (a top 50 middle) for the middleweight title while weighing noticeably less than him. McCallums body attack would be somewhat offset by the Kid's class, speed and countering. The more i think about it the more chance the Keed has.