Re: Could Marciano have feasibly went 77-0 in place of Dempsey?
If we take just Dempsey's known fights from 1914 - '27 (and there are probably a few missing in that time period anyway), and have Marciano do the exact same schedule, he's going to have to fight until an older age.
Marciano turned professional age 23 1/2, and last fought soon after turning 32.
On Dempsey's schedule if he turns pro at 23 1/2, he'd be about 36 1/2 against Sharkey and Tunney II.
Also, we have no idea how he'd do against most of Dempsey's opponents, who we just don't have much info on.
Let's not forget that Marciano had two controversial points wins in his own 49-0 career.
And three of Dempsey's recorded defeats, and several of his fights, were limited to 4 rounds, a distance over which Marciano may well have proved more susceptible to spoilers.