Hi, I believe the general consensus is its easier to get hold of an old Discovery box than to open the box up. I have been watching a few on ebay, but they seem to go for silly sums - especially the TD5 ones with diff-lock (later Disco's didn't have diff-lock or at least the lever to engage it).
As far as towing/pulling power - the independent dealer I bought mine from said that the transfer box swap was best done in conjunctions with a remap.