Done this a few times now. Just unbolt the trailing arms from the body, the hardline to the rear brakes, the handbrake cable at the adjuster & the shocks at the bottom & drop out the rear end. Get the H165 Stanza housing & put it where the old diff was. The new diff is slightly wider, but all the trailing arms stay in the same arcs. Some Stanza's have harder trailing arm rubbers, use them by preference. Use a trolley jack to raise the housing in & out, put the axle stands under the body, not the diff & lower the diff half way through it's travel as this allows all the bolts to be undone with no tension. can be done by one person if you can find this sweet spot. Make sure the springs are seated properly when fitting the new housing, as they can sometime sit on top of the spring perch or the upper parts. Just need to get a shorter tailshaft, which is coincidently the same length as a 1200 ute with a H165 diff. Unfortunately the Sunny tailshaft itself has peened in uni's, so unless your confident of the condition of the uni's, I wouldn't worry about getting it shortened. I've used Stanza tailshafts with the front yoke changed to an A series yoke, or picked up 1200 ute tailshafts when I could. Grahams wreckers at Smithfield NSW had a few 1200 ute bodyshells last friday, maybe a call to them would locate a tailshaft.
The Bluebird housing is too wide to fit under a Sunny, & I don't know if the arms line up. An S12 Gazelle H190 diff can be used with some changes that I can't remember, I think the lower arms are at a different angle. Was going to use the Gazelle diff under a Sunny Sedan with an L20B & Stumpy box, if I can find an LSD for it.