Here's the tale of the tape...

Jack Johnson
Height: 6'1 1/2
Weight: circa 205
Reach: 74"

Jack Dempsey
Height: 6'1
Weight: circa 190
Reach: 77"

I think that Jack Johnson in his prime had the style to deal with Dempsey but at the point in time you're talking about I think this unfolds like Tyson-Holmes.