If getting KO'd or beaten in wide decisions counts as hang with. Floyd might be able to run from Leonard, Hearns or Duran for 15 rounds. He'd lose wide to all three. He wouldn't be able to run from Robinson for 15 rounds though. Pac would probably get KO'd by all of them.
Past. The old timers you listed almost all were also in the PED era of mid 40's onwards. They were also in the golden age where skill and talent pool is concerned. In addition, they knew how to outpoint on the outside and fight incredibly on the inside in a way that will quite frankly bamboozle the new timers.
Robinson beats Floyd convincingly. Probably stopping him. Trinidad better not touch Griffith's ass that's all I'm saying. Armstrong would maul Pac on the inside. Depends on the ref with Zivic vs Norris. Zivic gets disqualified with a modern ref in charge. Oscar wouldn't be able to handle the pace Gavilan would unleash upon him and Oscar isn't cracking his iron jaw. Gavilan points in a one sided fight. Jose Napoles had immense skills. He beats every modern Welter with ease. Barney Ross is an ATG. If he can beat McLarnin and Canzoneri he's toying with the likes of Margarito.
a weight drained De La Hoya who hadn't fought at welter in years is not worth mentioning in the same sentence as Thomas "The Hitman" Hearns. I don't think Pac would last more than six rounds with Hearns. 6'1 to 5'6. The reach advantage would be huge, not to mention Pac would get hit by something that had far more power than anything Marquez could land on him.