My brother just put a w55 in place of a w50 in a RA28.
You'll want a 21R-C bell housing and only two of the gearbox cross member bolts will line up.
With a long strip of 6mm thick steel plate on each side and six accurately positioned holes in each you can effectively remount the cross member in the right place.
The front set of holes goes only into the floor pan and the flat bar, the middle set of holes has a bolt holding the cross member onto the flat bar and them going into the floor pan.
The rear set of holes holds the cross member to the other end of the steel flat bar.
Note: you way need to put a couple of small dents in the bottom of the floor pan so you can fit a nut of the end of the rear bolts.
I hope this all made sense, but it does work, and the Transport SA inspector didn't defect us for it, so it must be alright.