Tom hi mate i have put a few disco engines in defenders and i dont change fly wheel housing i just remove all studs apart from the top 3 then slide them together and refit studs that line up then drill and tap the others.
Use defender mounts on disco engine and use disco rad and intercooler with defender surround as you dont weld, you can use disco surround with some basic welding then a mixture of water pipes and a defender wiring loom or just the plug off the loom (i may have a plug at work)
I am in birmingham so not to far from you so if you need any parts or info P M me
Cheers and good luck
Billy