Some folks just aren't satisfied with a conventional diesel truck, so they swap in a 2 stroke Detroit Diesel. Parts are everywhere, they made them forever, and they come in small enough configurations to use in pickups. Just as much torque as the gas V8, but over 30 mpg in many cases.
People have used 2-53's (2 cylinder, 53 cu in per cylinder) all the way up to 4-71's.

Even a modern 2005 Durascrap was replaced with a tried and true DD.