I'm gonna try to pattern his weight classes with his actual career over a 10-11 year span.

As a young lightweight/super featherweight, I believe he would have beaten Arguello, Pryor and Mancini.

As a welterweight is where things will undoubtedly get more difficult for him.

He'll barely squeak by Duran (who was a tad past his prime by then) on points.

He'll suffer his first loss at the hands of Sugar Ray. Ray's power would be too much for Floyd in the 2nd half of the fight.

Hearns would have been his toughest opposition out of the bunch. I'd give Floyd winning 3 rounds tops. Hearns by UD.

Benitez: Once again Floyd would have tapped his way into a decision inflicting very little damage on Wilfredo
Late 80s

JCC: At this point in FM's career, he'll be going against a ripe Chavez. Chavez by split decision

Meldrick Taylor - this matchup would be entertaining as hell. However I believe Taylor's activity would give him the nod in a narrow decision.

When it's all said and done, Floyd would have had 4 losses by 1990. At least 3 for certain. I wish I could have elaborated more on the matchups, but I am posting from my phone. Feel free to add more fighters.