You'll need the BW66 converter to go with it, and the tailshaft housing is different so that will need changing. I think the tailshaft itself is the same length so that won't need changing, but if it does it's a gearbox strip to do it. You can use the BW66 rear flange but the bolt holes are very slightly different so if the P6 flange will fit then use that. There may be other little differences that will need bits from the P6 box but nothing major I can think of at the moment. The bellhousing is different, but not in such a way as it can't be used, but if you want it to look original then swap that over too. AFAIK the Rover BW66 isn't full 66 spec anyway as it doesn't have the deep sump.
