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Gunsmoke! "Cooter" (27 November 1954). Have to admit I can't help seeing William Conrad, 5'6" and 250 pounds (or so), as Matt Dillon facing down an outlaw, his plump little legs teetering on size 7 cowboy heels. John Dunning says Conrad would never talk about radio (on which he was nearly ubiquitous because of his resonant baritone), or CBS in particular after they went with lean 6'5" James Arness (aka "The Blood-drinking Carrot from Planet X") for the TV version.
Fibber McGee and Molly: "The McGees Go To The Movies" (28 September 1943). Typical wittiness and woidplay.
A very different show (tho also witty) from the long-time writer of Fibber, Don Quinn. The Halls of Ivy: "The Goya Bequest" (24 January 1950). [Update: Damn me if this one wasn't written by Ronald Colman, and it's as good as if Quinn had done it himself.]
Update: Caz sent along this video, which doesn't have anything to do with radio but is very, very cool. Apparently it was all over the interpubes (so immature) late last year, but I missed it, and so, therefore, did you.