It was 03 for the 2500/3500's, yes they do have more room, but IMHO, it's not a lot more. I had one for a couple years. Other than having an externally accesable door, I didn't find it a whole lot bigger than my 1999.
I know you didn't ask this, but for me, the ideal truck to me would be a new megacab with a longbed conversion. I've seen a few of them, and they are L-O-N-G... But I gotta have my long wheel base. I really regretted buying the 03 in a short bed configuration.