1) Are you sure the howling is coming from the diff itself, and not one of the outer bearings? Differentials usually last forever (unless they run out of oil), while the bearings do go bad. The bearings are retained on the outer part of the axles by a pressed-on sleeve - replacing them isn't a big deal. You might try turning each rear wheel, feeling for play, or listening for growling, to diagnose the noise a bit more.
2) All 2L Alfas can interchange differentials. So any '72 - 9X diff from a 4 cyl car will fit.
3) You lost me when you said "upgrade" - what kind of an upgrade are you looking for? 90% of all diffs out there will have a 4.56 ratio (your '74 almost certainly does). Later spiders from the EFI era had 4.10's, and I believe that spiders with automatic transmissions used a 3.XX. But, it's debatable whether a lower numeric ratio is an "upgrade" - depends on how you use your car.
San Juan Capistrano, CA
'63 Guilia spider
'65 Guilia Sprint GT