The gearbox will fit steaight to the old mounts as they are identical, except for the bell housing and primery shaft. The engine is a little more work though.
1. The engine mounts are in a diffrent place. You will need to fabricat and install new ones.
2. The exhast will need to be replaced with a Diesel version.
3. The heater pipes are all on the opposit side of the engine. You will need to make up replacments, going all round the engine bay.
4. There air cleaner is totally diffrent (paper filter on top of carb for Petrol, wing mouned air cleaner box for Diesel).
4. The ignition wiring will need to be modified to run to the fule pump.
5. The engine mounts on the block of the 2.8 may sit the engine too high (when lined up with the chassis). The feet of of the earlier 2.5 DG engine fit to the 2.8 block, and will allow it to sit lowwer.
6. The bottom radiator hose is on the opposit side. You will need a long hose going right accros th eengine bay, or a Diesel rad.
That's all I can remember of the top of my head. It was a few years back when I did this conversion. It took a morning to remove the Petrol engine, and get the Diesel bolted to the Gearbox and sitting on the chassis. It then took another three days to fabricat all of the other bits and bobs to make it work.