auto conversion

hi new to this go easy on me all ready had one dilema re clutch on defender 90 all sorted cheers for replys new question has any one fitted a autobox either from disco range rover want to know about props g box mounts etc also intend to do a lot of motorway driving in defender 90 85 plate like in a series if you change diffs you get more speed is it the same with 90 if not what is a realistic top end with a 200 tdi or 300 tdi td conversion all replys appreciated cheers