A 1980 Firebird would have an 8.5" rear end. The posi carrier from a 2.56 geared unit will not work with 3.08 gears. The 2.56 uses a 2-series posi carrier (cast 410409N), while the 3.08 needs the 3-series carrier (cast 410408N). In an 8.5, the 2.73 gears also use the 3-series carrier.
Gear tooth counts are often marked on the edge of the ring gear, usually right next to a date code. If you see four numbers right close to each other, for example 41 16 12 79, two will be tooth counts, & the other two will be month & year of manufacture. Divide the ring gear count (usually between 35 & 45) by the number next to it to get the gear ratio (41 divided by 16 = 2.56).
12 79 would mean made in December of 1979.
Novas from the early '70s (starting in '72) with 3.08's were fairly common, but their rear ends are a little narrower than the '70-'81 Camaro/Firebird's.
