[Pontiac]

Instead, FDR dumped Wallace and picket the much more conservative Truman to inherit the presidency.

Roosevelt could not stand Truman.

The Democratic Party leadership new that Roosevelt was not going to live through the 4th term and did not want Wallace to be president.

The party insisted on Truman.

[Maine Mariner]

Actually the choice came down to Truman or William Douglas.
FDR was asked who he wanted as VP. He sent a telegraph to the convention leadership that said Douglas or Truman.
Someone intercepted the telegram and switched the naems so it read Truman or Douglas. Truman, of course, received the Democratic nomination.