My point was that engine design is inevitably a compromise between various conflicting requirements. Automotive engines are optimised for automotive requirements, which, as you rightly point out, are quite different to those for a DEMU engine. A D78 engine will spend most of its life alternating between periods of constant speed, maximum power operation and periods at idle/shutdown. Maximum power means maximum mechanical stress and maximum component temperatures. Sudden cutback from maximum power to idle/shutdown causes high thermal stress, particularly on components like turbochargers. Purpose-designed railway diesel engines are optimised for high reliability and long intervals between overhauls with this type of duty cycle.
I might be proved wrong, but I think there is a risk that the automotive engine will turn out to be unreliable in the D78 application, and expensive to maintain.