in 69 the 2 stock diffs will be either a 8.2 10 bolt or a 12 bolt (count cover bolts). The 8.2 was made from 1964-1971 (some 72) so if searching the junk yards you'll need to stick to the older stuff. As far as your ratio goes either pop the cover and count the teeth and divide them ( I.E. 15 into 41 = 2.73 ). or jack up the rear of the car put it in neutral and rotate both wheels 1 rotation (in same direction) and count the drive shaft revolutions (I.E. 2&3/4 turns = 2.73)
if you can find a complete diff from another nova that be a score. I used a 8.5 ten bolt from a 72 SS that had posi and 3.42's. The spring perches are different from mono leaf cars to multi leaf cars too.
If the rear is original it will only be either an 8.2" 10-bolt or a 12-bolt. The 8.2" 10-bolt isn't made of glass but it is weaker than a newer 8.5" 10-bolt. Any 1972-75 Novas will have a 8.5" 10-bolt and will bolt directly in. Some 1976-79 will have 8.5" 10-bolt but you have to check because they might also have a 7.5" 10-bolt.
I've never seen a 72 with an 8.2" but I guess a few early ones may have gotten one. And while 3.08 is very common for 1972-75 there were quite a few 2.73 and 3.42 ratios.
As for 1976-79 Novas, not, it was not V8 only. Especially in 1976, the 305 got the 7.5" 10-bolt. And I've been told that some 250 L6 Novas those years did get the 8.5" 10-bolt so don't assume. You gotta check.