My instinctive thought is Harold getting KO'ed. If they fought a series, perhaps Johnson would put on a boxing masterpiece and take a bout, but more often than not, I think Spinks just hits too hard and sharp for him and finds the target and destroys it. Interesting fight though. Would love to have been able to see it.
Johnson had the ability to upset anyone at 175,but was a counter puncher,who let the other guy lead,Spinks herky jerky style while not attractive is hard to read ,I think Harold would be too passive to win this."Harold makes you do all the work"Archie Moore.
I'm not sure it would even be a upset. Harold Johnson might well enter the fight as an even proposition or a slight favourite. Look, he had wins over Archie Moore, Eddie Machen, Nino Valdes, Ezzard Charles, Clarence Henry, Doug Jones, Bob Satterfield, Willie Pastrano ... and others.
I agree that it wouldn't be an upset really. Johnson would be the best LHW Spinks met for sure. Still pick him though.