Fitz certainly did cut Jim up something good, but must be noted that Fitz had wrapped his hands for this fight - not common practice for the times, so his punches were no doubt more powerful than previous.
Plus sure Fitz broke a hand, or both, on Jeffries head, due to Jeffries habit of crouching and taking blows on the top of his head.
Corbett was a name (money) fight, and sure their last contest was a retire fund raiser for Corbett. Jeffries has all the names from the period on his record.
I'm not sure in his prime there was a limit to how much Jeffries could take.
I like to use Marciano as a starting point when describing Jeffries' character. I think an opponent would have to kill him to stop the fight.
He was inactive for 6 years prior to the Johnson fight, and needed to shed in excess of 100lbs. Throw in the pressure on him when he had little-to-no-chance, and Johnson although it was his style, still took some time to render Jeffries weak enough to put on the canvas.
I'm still not saying Jeffries would be a dead cert to beat either Klitschko, but when they got into that ring, if their skill sets were bridgeable - Jeffries would win.
We can't say that Fitz had his hands wrapped with an illegal product. We just don't know for certain. It is heavily rumoured because of how much damage his punches did compared with the first fight. But it most be noted that Fitzsimmons was spotted drunk a day before their first fight, which he didn't seem to take seriously. Plus Jeff gave him more than enough respect and didn't charge forward as much as he did in the second meeting.
Jeffries does have all the names on his record (Choyneski aside), but it was an incredibly thin talent pool to choose from IMO. As I've said, Fitzsimmons and Corbert were both past their best, Sharkey had been destroyed by Fitzsimmons. Ruhlin is likely one of his better wins, and he isn't a great heavyweight.
There was definitely a limit to what he could take. Don't get me wrong, he had an incredible chin, and I don't put too much emphasis on the Johnson fight given the circumstances (even then, Jack was toying with him and could have stopped him anytime he wanted IMO). But a 170lb or so Fitzsimmons laid an absolute beating on Jeffries. You could only imagine the damage a 250lb Wlad could potentially do.
How the fight goes:
I find it hard to imagine Jeff slipping that jab if I'm honest. I have all the footage on Jeffries, and he doesn't look great at all. I'm not sure he has the footwork or head moment to do so. My problem would lie when Jeff does get inside. Wlad would look to spoil (which he is pretty good at). Jeff might be physically stronger, but I dunno if there would be that much in it given the weight of both fighters.
I also think Jeff's power is a little exaggerated. He couldn't get rid of Choyneski (who Fitzsimmons sparked), nor Sharkey who Fitzsimmons stopped also (despite the bullshit DQ). It took him until the 24th to stop Corbett who Bob again accounted for, 10 rounds quicker.
Jeff was far from unbeatable in his prime. He was comprehensively outboxed by Gentlemen Jim in their first fight, and Fitzsimmons had him on the edge also. Jeff used his physical attributes to catch up with both eventually, but he couldn't rely on them against a 6ft 7" Wlad who not only outweighed him, but who likely hits harder and has one of the best jabs in HW history.