If I might offer my two bobs worth: try an overdrive behind an Isuzu and see how it goes. I have heard a lot of theories about the longevity or otherwise of various Landrover transmissions behind Isuzus eg an R380 won't last the distance - yet when I speak to people who are driving vehicles with the various transmissions fitted I find out that they have not had significant problems. So try an overdrive - in good condition - drive with a bit of mechanical sympathy and we might all be surprised at how long it lasts.

MyR380 seems to handling a 4BD1T without any problems (over 20000ks now - heaving towing - long distance fully loaded) - the torque rating of the box is significantly above the torque output of the engine.

Christopher