There are many candidates for this extra spot: Davis, Giambi, Chavez, Chapman, Miller, and several of the extra starting pitchers. I'll go with Harry Davis – after all, he was the first person to lead his league in HRs in four consecutive seasons.
With so many talented players, and no clear cut best of the bunch, it is hard to choose one to be the captain. Grove and Plank were outstanding starters, and for a few years Eck was unhittable. Simmons and Foxx put up some insane numbers. But I'll go with Rickey Henderson as the honorary team captain of this all-time team because of his value as a leadoff hitter and his all-out dominance on the basepaths.
Starting Lineup A mythical starting lineup for this squad might look like this: Rickey Henderson OF Eddie Collins 2B Al Simmons OF Jimmie Foxx 1B Reggie Jackson OF Mickey Cochrane C Sal Bando 3B Bert Campaneris SS Lefty Grove SP
This is an outstanding lineup of course, starting with arguably the best leadoff man of all-time. And he is followed by several high-average hitters in Collins, Simmons, Foxx, and Cochrane, and several with power in Simmons, Foxx, Jackson, and Bando.
Of the six outfielders on this roster, only Murphy played predominantly in CF. But while both Henderson and Simmons played more often in LF, they each played CF for a few seasons, so you could legitimately slot either of them there, and that way get McGwire or Johnson in the lineup as the DH.