I don't disagree with some of your points, but do disagree with some of the conclusions you've drawn. Take for example the above, the decent amateur fighter, with some good competitive experience, Fitz would eventually KO, and possible hurt said fighter. You can't teach the timing/reflexes/instincts he possessed, all the while generating power few have ever rivaled; and then expect me to believe he's not going to set a trap that eventually ends the fight dramatically, and instead, will some how be bewildered by combos and a modern style, and I don't agree, at all. Size is a variable, but against a great fighter, you need to be more than just big and modern imo.
Things that should not be in question about Fitzsimons, are his power and ability to end a fight. These would be enough to make him dangerous, even if he was as bad as some people claim.