Here are some videos of a few Datsun 240Zs with diesels shoved in them.
This one uses an OM606 Mercedes turbodiesel:
This one uses a 2.8L Datsun diesel that is a bolt-on fit:
I couldn't find mpg figures for either, but they are probably at least in the high 30s when driven with a lead foot.
They would be formidable on the highway where the engine is operating right where its max powerband is just for cruising...