It would have been fun seeing him fighting Ezzard Charles, Archie Moore, Lloyd Marshall, Joey Maxim, Billy Conn. If Spinks remained undefeated, he would be incredible. He probably would have also fought as a heavyweight.
I think he’d have still done very well and been one of the top light heavyweights of the era, but given differences in scheduling, the frequency of fights and the overall quality of competition there’s no way he’d have remained undefeated. That’s not a criticism of Spinks, just a reflection of how different an environment it was to fight in then.
wouldn’t have happened in those days. Not only would he be up against the stiffest opposition that division has ever seen but he’d be fighting triple the amount of fights or more. Nobody got out of that 175 lb era undefeated.