Johnson's style was different than that of others of his era in terms of head movement, footwork, and defensive skills in general. He was a pioneer, and I think we would've been able to hang (without "modern" training methods) with today's heavyweights quite easily.
40 odd years ago, Jimmy Jacobs collected a large number of old boxing films. He compiled some of the highlights of those films and showed them to the boxing writers of the time. The writers were shocked at the relatively primative styles of legends like Jim Corbett, Bob Fitzsimmons, and Jim Jeffries. One writer said, "I didn't know whether to laugh or cry."
It was the consensus of the writers that of that group, only Jack Johnson could have competed with modern fighters.