Sorry Jim. . . there have been a couple people I've heard of who have custom-fabricated what you are looking for. I am 100% certain nothing is available commercially like what you want.
I know the 4-cylinder version of our Cummins is available without turbocharging, but the 6-cylinder has never been offered non-turbo that I'm aware of.
There has been discussion on TDR before about setting up an engine with a supercharger instead of a turbo, but no one has done it. Party due to a supercharger is not as efficient and it is working all the time whether or not you're getting on the throttle. Turbos work so well on diesels there's not much point in doing it another way.
I have wondered what a naturally aspirated 24-valve could do with a serious head porting, cam and injectors, exhaust headers and open exhaust. Nowhere near the HP as a turboed version, may not even produce as much HP as a stock turbo Cummins, but for someone with time on their hands it might be fun to try.
Another guy on NW Bombers built a split manifold and ran 2 small turbos in parallel as an experiment, HY35s with 9cm housings, but it ran terrible. . . total dog.