found a rear not sure what it is? need help

[CHUMPCHANGEBU]

found a 10 bolt with casting #'s 3969277NF when i look them up it shows 70 thru 72 chevelle with 2.73 positrac. dont want to get burned when did 8.5's start coming in chevelles and can anyone tell from these pics
any help would be great thanks again

 

[LS6 Tommy]

The numbers are for 70-72 Chevelle but the classic "squares" on the bottom of the case aren't there, but I'm thinking the numbers tell the story & it's an 8.5". Do the casting ribs on the side of the cas converge or are they parallel? If they converge it's an 8.2". If they're parallel it's an 8.5"

Tommy