Paul Shenar (whom I know from hardly anything else) made a very good Orson Welles. And of course Caset Kasem in his big scene as the radio actor.
The movie clears up one thing (something that might be common knowledge to WOTW fans, but I never knew it), and that's why the Secretary of State (or whoever it was) sounds the way he does on the show. Evidently the show was going to have FDR address the country but they were told they couldn't use HIM in the story. So the actor just did his FDR imitation under another name.