I am pretty sure it wont. I am in the middle of putting a 200tdi disco engine in a 1989 2.5 petrol. I have taken loads of pics so will write that up when im done. The disco box would have the lever in the wrong place at best. I would think you would need to make or move the geabox mounts, modify the tunnel where the sticks come through the floor and possibly change the props?
You should be able to get a second hand box cheap which would be much less effort. I just sold a j reg 200tdi disco (with empty engine bay) to local scrap yard for £120! I got £82 on ebay for my petrol engine and rad etc. That might help when you are adding up the cost of a conversion...
Cheers,
Mike