I do not believe I have enough information to comment on the two nominees whose careers were spent in the Negro League, but I will say the statistical documentation I have found on Baseball Refence.com is limited with regard to Vic Harris (10.6 career WAR) and John Donaldson (3.4 career WAR). That's obviously not their fault, as racism kept them from playing in the American or National Leagues, but I am not comfortable evaluating their careers.
I also understand that there are at least two different methods of calculating WAR, neither of which is Holy Writ, but I do find the stat to be useful. That said, as for the other nominees, their career WAR is the following:
Luis Tiant 66.0 WAR (44th all time for pitchers)
Ken Boyer 62.8 WAR (14th all time for third basemen)
Tommy John 61.5 WAR (77th all time for pitchers)
Dick Allen 58.7 WAR (23rd all time for first basemen)
Dave Parker 40.1 WAR (54th all time for right fielders)
Steve Garvey 38.0 WAR (51st all time for first basemen)