Well a little knowledge of LR model history and configurations as well as ROW vs. NAS set ups might have helped you here Rob because this is a totaly different animal to what you did with your modern D2. Remember this is a non NAS 1983 110 getting a NAS 1995 Disco set up. If we had access to a NAS 1993 110 Harness then it would have been much easier but they are NLA so we had to start with a new 1983 110 harness and go from there. The mechanical side is partially easy as has been mentioned with the removal of the clutch mechanism and pedal. After that the fun begins.
1st problem is the NAS part. NAS trucks have shift lockout solenoids fitted to the Trans and T-Case that ROW trucks do not. Then there is the starter lockout circuitry. Trans and T-Case Temperature sensors and indicator lamp that where not present on a 1983. Etc., etc. The 1983 harness is very simple and has none of the curcuitry to support a NAS Autobox (even a non electrical one). All that has to be added, spliced in or modified.
From a mechanical standpoint the major problem is the shifter set up. NAS shifter and console set ups are NLA. So the Ashcroft version is the only one available 'off the shelf' as you put it. But its wasn't designed for a NAS so it has to be modified. T-Case linkages are different and have to be switched around. Trans Tunnel was designed for an LT95 so has to be modified, modifying the trans cooler lines and mounting a trans cooler, and so on.
So no, not an off the shelf undertaking. Not overly complicated but certainly not 'drop in' by any means.
Remember that there are manual trans D2s out there and you where able to source the parts you needed off the shelf and simply diasable the electrical circuits you didn't need. Your biggest issue was probably programming. There never was a factory NAS ZF Auto 1983 110 so its a little more difficult.
Thats just the Trans. Now throw in in the 3.9L EFi Engine part, the installation of the TD5 Dash and instruments and so forth and you start to see that this is more of a total rewire and rebuild of the basic systems.
Combining parts from 2 or 3 trucks built for different markets 12+ years apart is 'off the shelf'? No, not so much.
I'm guessing it's about as hard as swapping an R380 for the ZF. Which is to say, not hard at all. All the parts are off the shelf.