The 1500 Tradesman HD was a one-year-only vehicle if I remember right, basically an experiment. They never built anything but regular cab, long-bed versions, and their use of the front clip from the 1500 precluded the installation of the Cummins. I expect that had there been a diesel that would fit under the 1500's front clip, it may have been available in such a truck, but the truck is an aberration in that it could serve to cannibalize sales from the more-expensive 2500 line. That's probably why they never offered regular cab, short bed or any crew cab version.
If I ran RAM, I'd offer the 1500-type front clip on any truck whose powerplant can accommodate it, basically as a no-cost option. Anyone not buying a Cummins truck could have a nicer look that way.