I suppose you mean the Alfetta Sedan, not the Alfetta GT/GTV/GTV6? There were four-door Alfettas with turbodiesel engines, but I seem to remember they were the later "longnose" version. Grafting a TD VM engine and all its ancillaries in a Sports Sedan, however, might be more difficult or impossible because of lack of space - maybe more seasoned European BB'ers could shed some light on this?. Regarding weights and so on, I don't see why it should not be possible, it is practically the same chassis (although the specification/thickness of the torsion bars in the Alfetta TD version may different from the ones used in the 2.0 Otto engine).
What about the utimate perversion - a turbodiesel Alfetta GTV/GTV6? And what about an automatic GTV6 - I understand it has already been done Stateside, using automatic Milano bits?