My feeling is that McLellan gets to Hearns in the the mid rounds but i think it's equally possible Tommy wins a decision.
If Hearns boxes smart and clinches like he learned to later in his career, he beats McClellan... he is simply a far better fighter
He wont survive McLellans bombs for too long, even if he clinches. This is one of those unusal cases in boxing where the better man would likely lose every time...simply because he one weakness will be exploited by one of the major strenths of his opponent.