It could go either way. The upper limits of Marciano's durability were never tested. Moore or Walcott was the best punchers he fought and both floored him. Neither was in Fitz's class as a puncher. Rocky also had issues with cuts, something Fitz was good at creating, but part of that could be the gloves they used in his time. From 1890-1904, nearly 14 years of boxing Fitzsimmons fought the best punchers out there. Only one man stopped him, and it was Jeffries, so Fitzsimmons durability might be close to Marciano's. IMO, Fitzsimmons fought the better punchers. Marciano, of course, was never stopped, but he did retire at what can be viewed as a young age. I think Fitzsimmons has the power edge and speed edge, while Marciano is the better fighter on the inside with a more modern technique in terms of defense. 50/50
Rocky see those spots and faints. Never even gets into the ring. Seriously Fitzsimmons looked like he had leprosy.
Fitzsimmons by KO eventually, he’s a murderous puncher and he thrived on bull-like fighters like Rocky, who is also quite hittable. We can already see that Fitzsimmons has the speed and power from the film, but he also apparently had excellent timing and was regarded as a pristine finisher. I don’t think his awkward performance in the Corbett reflects how he does against Marciano