There is some info in this thread, if you move back and forward through it.
Harrison diff setup
There are lots of Nissan aftermarket spares places that have the upper arm replacements (some are 'U'shaped and others round for the coil over to pass through). They are less expensive. I am surprised that the guy you spoke to did not know about R33s.
I can't see how they would be an engineering or inspection problem, but check with the engineer that you have lodged your intention to build paperwork with (if that is the way its done in NSW?).