I have the option of a complete and running low mileage 3.9 v8 auto Disco
The fist thing to say is DO IT!!!!
You will become hooked on the smooth power delivery from nothing and then there's the noise
As some folk will tell you its not the exhaust note you hear, its the fuel pump emptying the tank...LOL
But to be serious, The conversion is simple but keep the disco 'till the jobs done and of course you also get a free disc braked back axle...
Of the top of my head you will need the following from the Disco (in addition to the engine and box):
Front exhaust Y piece
Front Xmember (bent tubular one)
complete wiring loom from the engine to the ECU and both half's of the connectors to the disco body loom (located beside where the clutch pedal would be). Take as much of the disco loom as possible as it makes it easier to integrate into the 90.
Engine mounts
gearbox mounts
Autobox shifter and cable. When you remove it, disconnect at the gearbox end and it makes the refitting easy!
Depending on how old the disco is, it may have cats in the Y piece, these can be removed but you should change the "tune" resistor so the ECU knows not to look for them.
You will also have to modify the transmission tunnel and seat box to "create" clearance. You can use a 300tdi or TD5 bulkhead plate, transmission tunnel and seatbox (if you can find them!)
Hope this is of some help and if you are near Edinburgh and would like to see one that was done recently, let me know.
Bob