I didn't follow it real close but this thread may provide some answers:
http://alfabb.com/bb/forums/showthread.php?t=18020
Also, I think 4.55 (or 4.56?) LSD rear ends were standard on all U.S. bound Spiders, Berlinas and 115 GTVs from about 72 - 80 (give or take a year) so an American supplier might be a good source for your ring and pinion. Good luck.
Bill