Let's suppose that Duane Bobick never went to South Africa to fight and lose to Kallie Knoetze in February, 1978. Also, let's suppose Leon Spinks, after beating Ali for the title that same month, is free to make a voluntary defense of his WBA belt. Spinks selects Bobick as the opponent.
Bobick is 40-1 entering this fight, and Spinks is 7-0-1. The physical stats are Bobick at 6'3", 214lbs., and Spinks at 6'1", 201lbs. Both guys are at their best.
I would rate this bout at a 60% chance for a Bobick victory, and a 40% chance for a Spinks victory. It would be a very "iffy" kind of fight.
The reason I give Bobick the edge here --something that may surprise many-- is because of the trouble Leon always had with physically strong fighters who could bull him around. We saw Spinks struggle with Scott LeDoux in the fall of 1977, for instance. Bobick was stronger, more technically proficient, and harder hitting than LeDoux.
In Leon's favor is that Bobick was a slow starter who didn't take a big punch very well. Leon wasn't a huge bomber, but he had snap in his blows, so there's the possibility that he could gotten Bobick out of there early.
Here are the two scenarios I envision...in both instances all of the action takes place on the inside...a classic "barroom brawl":
SPINKS VICTORY (40%)
Leon starts fast in this fight, winging in quick, crackling left hooks and right hands on Bobick's jaw. Leon is on top from the opening bell, and Bobick just doesn't seem to have the agility or the speed to stay out of harms way. In this case Bobick could be overwhelmed in round one, or he could have been beaten down and stopped within four or five rounds. This is a close quarters fight, with Spinks's superiority in speed, aggression and firepower carrying the day.
BOBICK VICTORY (60%)
In this scenario, Spinks gets off to a more aggressive start than Bobick, and he wins the early rounds with his crackling combinations. Leon, however, just doesn't seem to have the overwhelming kind of punching power that is necessary to exploit Bobick's durability problems, thus Duane remains on his feet in the first few rounds despite falling behind on the cards. By the fourth or fifth round, Bobick's strong, clubbing body attack is hurting Leon, and the tide of combat turns in Duane's favor. Leon's punch output drops as he is clearly being hurt and bullied by the bigger, stronger Bobick. Bobick shifts his attack upstairs, and by the eighth round he's won the fight by kayo or TKO. Leon makes several trips to the canvas.