I too had done a fair bit of looking into a similar swap for the Disco. I was looking at a simple task with a D1 but in the interests of longevity you reall do need to put the GM transmission behind it.
Apart form the costs in actually doing the work itself, the engineering compliance is a bit deal. Emissions compliance is the largest factor of the lot in getting it to work. You have to use the GM ECU with the setup as the engine was set for compliance that way. There is no way of replicating the ADR test so you have to emulate what was with the engine when it was sold. That means you'll be fitting catalytic converters etc. as well.
After all that I figured a nice 4.6 would be much simpler and easier to achieve, and it is. I've got the engine sitting in my garage while I get everything else together to make it happen.
