What twenty six foot box truck is known to run well over 350000 miles?

The length doesn't really tell you anything. For current ULSD fuels, you want a more recent model - ULSD will shorten the longevity of engines manufactured before ULSD (Ultra Low Sulphur Diesel) became a requirement.

Other than that, we'd need more specifics as to what you're looking for, what Gross Vehicle Weight Rating you're looking for, etc. You can get a Freightliner M2 with the 6.7 Cummins, and, as long as you do your regular maintenance on it, you should have no problem exceeding 350k.