I grew up with James Arness as Matt Dillon and we never missed the TV version if we could help it. As good as he was it was hard to imagine anybody else playing the part even though he was the second actor to do so. Having heard William Conrad's portrayal though I can see why so many of the fans of the radio show were outraged when the news came that he wouldn't continue the part on TV even though he didn't physically match the character. Conrad is so incredible in the part that it would have been hard to believe that they could get another actor who could be as good. That they did and the TV show continued the high quality of the radio show is one reason this series ended up being one of the longest running series of all time.
The Internet archive now has the original radio show available for free. At the link above you can either listen to it via streaming audio or download episodes to listen to later. I've been downloading them and listening to them during my commute
I can't say enough about how well done these are. The very first episode, Billy the Kid, gives away a key plot twist right in the title and still managed to give me chills when the reveal happened. They include richly layered background sound so you feel like you are listening to something recorded on location and not in a studio. The stories are complex with characters who tend more to the grey than being purely good or evil.
That last was intentional. William S. Paley, the head of CBS, wanted a Western for adults and originally requested that it have the same feel as the Phillip Marlow hard boiled detective series did and even had the developer of that radio series working on the project for a while Gunsmoke got shelved for a bit before another team got interested in it and the end result isn't a Western Phillip Marlow but just like the TV version it's not a Lone Ranger sort of thing either.
The characters we know are a lot darker back at the beginning. Doc exults when a lot of people get killed in a gunfight because he needs the money. Kitty doesn't show up for the first few episodes and when she does she isn't the confident saloon owner we know from TV but just one of the girls who works at the Long Branch. Doing what? It's never outright stated in the show because of censorship but it was verified in interviews that she's a prostitute. Both would develop into characterizations closer to what we're familiar with from TV as the series went on.
Norman Macdonnell, who was in charge of the series, would have been in his late thirties when the radio version began in 1952 and like many his age and older then could have heard stories as a kid from people who had been alive in the 1870's when the series is set. To put it into perspective that time period was to people his age what WWI would be to us. He made an effort to make the series as realistic as possible in feel even though they did obviously have to take liberties with some things.
I highly recommend this even if you aren't usually a fan of Westerns. Good well written drama is good well written drama no matter what the genre.