Disc diff needs leaf spring mounts plus a different handbrake set-up. I think there is an issue with the calliper clearance and I think they swapped the callipers side to side or something. It's been done plenty of times though so can't be too difficult.
There is stacks of good bits you can use out of the Statesman. Kick panels with footwell lights. Integrated air but if you use a GTS dash you will need the correct AC controller. Seats but as mentioned WB has different bolt pattern. You can effectively put the complete Statesman in the HX.