The table below shows his splits. Data from the Baseball Reference Play Index
Now the league splits from 1936-51
So players normally had an OBP that was .015 higher at home and a SLG that was .021 higher. What if DiMaggio had played in a fair park his whole career and he had these same differentials?
His home OBP and SLG would be .420 and .631. If those are averaged with his road numbers of .405 and .610, he would have a career OBP of .413 and a career SLG of .621.
That is better than his actual numbers of .398 & .579. So his OBP goes up .015 and his SLG goes up .042. That would raise a team's OBP and SLG by 0.0016 & 0.0046, respectively (assuming he has one ninth of a teams ABs and PAs).
How many extra runs would this mean? I ran a regression with runs per game as the dependent variable and OBP & SLG as the independent variables for all MLB teams from 1936-51. Here is the equation
R/G = 11.19*SLG + 19.20*OBP - 6.17
Plugging in the 0.0016 & 0.0046 changes in team SLG and OBP, we get 0.0825 more runs per game or 12.7 per 154 game season. That is about one extra win per season.
DiMaggio played 1736 games. That is 11.27 154 game seasons. That times 12.7 is 143. That adds about 14 to wins to his career value.
He has 78.2 career WAR, good for 41st among position players. This adjustment would give him 92.8, putting him at 28th.