Referring to Alfa as Alpha is guaranteed to get you shot at dawn.....
The 101 Giulia & 101 Giulietta diffs are identical, except for the ratio's. The '63 to '65 Giulia Normale's used a miserably low diff ratio of 8:43 to 1 or 5.25 to 1, rather search for a standard Giulietta Normale diff of 9:43 or 4.555 to 1, coupled with a 5 speed box in your Giulia Spider, you will have a much more user friendly car
The 1300 Giulietta and 1600 Giulia rear axles are interchangable complete, but are not the same internally. 101 1600 axle bearings, diff. bearings are all different. 1600 bearings and axles are beefier. Axles are the same as 105 1600 and 1750. A 101 Giulia Spider came with a 5.12 ratio and the 1600 Veloce with a 4.56.
A 105 1600/1750 rear axle can be combined with the trumpet side of your 101 diff. with some set up to get a 4.56 or 9/41 ratio. You stock ratio is an 8/41. Great for racing. Not so great for the street.