Each member of the radio cast received an audition for the TV series and all were quickly rejected. James Arness received a big boost for the starring role from his friend John Wayne; Arness was under contract with Wayne production company at the time. The Duke thought highly of him and even introduced the series on CBS.
Ironically, Howard McNear (Doc in the radio series) made guest appearances in several episodes of the TV version, and William Conrad directed three episodes of the TV series.
I don’t know how well Gunsmoke would have fared on TV with Conrad, there weren’t many portly TV cowboys in the 50s. Maybe in comedy TV he might have worked, e.g. Gleason.