Fitzsimmons was skilled in the techniques of getting leverage behind his punches, in-fighting, body punching, and feinting. Most of these techqunies are gone today. There is only 20 seconds or so of clear an up-close film on Fitz, and he does not look as primitive as you say. The way I see it, is rate the MAN, not the time he fought in. If Fitz was in Monzon's time his technnique would look different. Likewise, if Monzon was in Fitz's time, you might call him " embarrassing ".
That may be the case, but it's not the question I was answering. We're not asking who'd win if both fought in each fighter's era with the same training and facilities, we're asking who'd win prime for prime, based on what they accomplished and how they fought during their time. In that case, a fighters of Fitzs's era, and Fitz himself, as the film suggests, just doesn't match up to a fighter of Monzon's caliber under a modern ruleset, which I specified in my original post.
Fitz by kayo in the 14th round of a real barnburner. Fitz suffers two knockdowns and is a bloody mess before he finishes Monzon in the next to last round.