I think he'd have struggled mightily against the Four Horsemen of the Apocalypse, and especially against Tommy Hearns.
Pryor would have desroyed him at light welter and maybe Duran too, I think would have fared better vs Leonard at welter but Hearns would have been far too big, by the late 80's he would have found a weak light middle champ to beat while ducking McCallum and Jackson, he would have won a lightweight title though and a super-feather title.
Interesting one. With same day weigh-in I think he still wouldn't start higher then 135, max. Quite likely 130, actually. He never cut much water weight. So rather JCC than Pryor as first really big fight. Nelson could be some time after that. Then there's Whitaker at 135. By the late 80's Floyd might start to grow out of 135, but somewhere down the line they'd probably meet. By the early 90's he should be at WW. Trinidad is on the rise and DLH will join the division in a few years, then Mosley. I think he faces at least one of them. The money is too good to say no to. Especially for DLH. But this is a gauntlet to run, through. Hard to believe he keeps his 0 with this kind of comp. He'd have the chance to actually put together a GOAT resume if he did beat these guys, though.
Seven years into his career he was 138 lbs in the ring, against Castillo. He'd never be anywhere near Hearns's weight if he started as a pro in the early 80's. He'd just be arriving at WW around the time Hearns won his second title at 175. He'd have good reason to sweat at the thought of facing Hearns there.
I think he still ends his career as an ATG but he picks up some losses this time. I don't see him fighting the Leonard, Hearns and Haglers of the time, partly because they're too big and too risky, and also because, assuming Floyd goes pro in 1980, by the time he becomes a top guy in 130/135 Leonard and Hearns wouldn't even be at 147 anymore. I see him going 2-1 in series' with Aaron Pryor and Julio Cesar Chavez, aswell as picking up wins over Arguello, Camacho, Rosario, Ramirez and his uncle Roger. Even without Leonard and Hearns at 147 I'm not sure he'd risk a 147 move at this point, not when a young pre Honeyghan Donald Curry is tearing through the division. I could see him moving up for a one off fight at 154 against Duran around the period of the Davey Moore and Hearns fights, in which case I think he wins against that version of Duran. Later on in the decade I do eventually see him moving up to 147 and picking up a title there against someone like Honeyghan. Might fight Starling aswell who I think he beats in a close fight. He might take on Pernell Whittaker in the late 80s or early 90s or he might delay the fight a few years. If they fight in the late 80s I see a another 2-1 trilogy for Floyd, but the later it gets the more likely it is that Floyd gets a win over him and leaves it there. For the last stretch of his career in the mid to late 90s, I think even with the additional wear and tear he'd have picked up from earlier tough fights against Chavez and Pryor he'd still have enough to beat Tito Trinidad in a decision. That's *if* he takes the risk of fighting him which is definitely not a guarantee. He might choose to retire on top after either beating Trinidad or beating Sweet Pea, but if not I see a reversal of what actually happened. He takes on a younger upcoming Oscar De La Hoya and loses despite putting in a good effort and ultimately retires after the match. Though Oscar might give him a rematch in which case I think he ends his career with back to back losses to De La Hoya. He ends his career as either a 4 weight champion or a 5 weight champion if he can get to Duran right after he beats Moore. would have something like 3-5 losses on his record.
I think he would have lost to Duran, Leonard and Hearns. As it turned out, he was around at the right time. He would never have fought Hagler on account of kefalitemnophobia.