I was listening to the Star Wars Radio Drama last night, and it was good stuff. However, the one thing that stuck out to me was that Darth Vader's voice was not nearly as intimidating, and often came across as somewhat cheesy. If you were to dub over JEJ in War of the Stars with his "soundalike" from the radio drama, I think that the effect would seem a lot more believable.

The best option of all would be to use David Prowse's on-set dialogue, but unfortunately I think that Lucas is the only one who has access to the whole movie's worth of them so I don't think that would be too plausible.