Converting single range box to dual range?

[wagonist]

I've got a single range, full-time 4wd gearbox with centre diff lock at home.
I also have a part time 4wd dual range box.

Anyone know how hard it is to convert the single range box using the dual range parts?

Bits I think I need to change:
RHS gearbox casing (has low range lever)
input shafts
if the gear ratios don't match, then 1st, 2nd, reverse on the output shaft

All sounds easy, but...