Well, IMO Fitzsimmons chin wasn´t good enough to survive against one of the hardest punchers ever (IMO Mac Foster is powerwise very underrated), Mac´s chin on the other side was pretty good. Fitzsimmons would knock many big very good HW´s out, but Foster would beat him IMO...
Mac Foster never beat a top ten rated fighter,he could hit and Fitz was dropped by Corbett ,an accurate but not hard puncher,difficult one,Fitz would be conceding tons of weight ,but he did that many times,it took Jeffries 12 rounds to beat him and Fitz was 37 and not in the best of condition,I have a slight leaning towards Fitz .
Fitz was a COUNTERPUNCHER, that was his MO. Bob could stand up to Jeffries(for quite a bit anyway) and Ruhlin, took Sharkeys, Mahers(just) and Choynski's punch, so I have to favor bob, big time. Suppose Mac has a punchers chance...
