I have heard that there is a Legacy rear shaft that has an R180 spline on the "inner" end and an R160 spline on the "outer" so you don't have to do any of this.
The person who told me this was a Subaru parts importer and just happened to stumble across this anomaly.
Unfortunately I cant remember exactly which Legacy model it was though :-(
The Later Spec B Legacy uses a R180 diff but i believe they still don't just fit on
The legacy shafts are longer
The diff on a legacy is Helical and i believe the Impreza is Mechanical? does that make a difference in fitting?
You may be able to use the Legacy diff but i think they at 3.5 ratio
Above is what i have read in the past and may or may not be correct
I would like to know 100% as sat in my garage I have an R180 diff, hubs and (brakes - poor condition) but no Driveshafts
and would be nice to just swap the diff and shafts
The guy who told me shut up shop and I'm not sure where he is working now. I bump into him every now and again at rally's so if I see him I will get the exact details from him...sorry I cant be of more use.