The ten round distance suited Dempsey over 15 he would have been stopped. Ali couldn't retain his pre exile peak and Marciano damn sure wouldn't! Marciano did'nt lose his hunger for a prospective bout with Johannson he lost his desire to sacrifice enough to get himself into the condition he needed to be in. It would have been the same against Tunney.
If I follow the timeline as suggested in the OP, Marciano goes into "exile" about the time of the Louis bout and comes back about the time of Charles II. I think he would be fresher, less wear and tear and plenty young enough, given his work ethic, to get in peak shape. He was 36 (?) about the time the Ingo bout was suggested, which does not follow the timeline as I understand it. Tunney just proved very little at heavyweight. I'm not going to pick him over even a compromised version of Marciano, even if we disagree on the the degree of that compromise.
IKR??? Gazelle and i, the leading rockistas, agree that Tunney would win, but SEAMUS of all people is championing Rocky in this thread Goes to show you he hates anything associated with Dempsey even moreso that Marciano
Tunney's heavyweight tenure does not impress me very much. He played a quick game of capture the flag, defeating non-entities late in their careers, seizing the prize at an opportune. I think that Dempsey had peaked quite a while before he fought Firpo and had feasted on set-ups for years BEFORE his "exile". Thus, he wasn't sharp and getting sharper as was Marciano at the time of Louis. Also, I tend to favor Marciano's chances against Dempsey more than most here, tho that is not 100% relevant.
Is that Genaro Hernandez in your avatar? I really liked him. A really decent egg and a very good fighter.
Mia Culpa... I read your comment as "become a great heavyweight". Spinks was good. Moore was good for his era. Jones, Jr was an opportunist like Tunney.