well I called my Gm dealer with the number off the axle housing and mine is a 2.41 so that is series 2. I have to change gears cus I just put that new engine in and am losing ALL my torque with a 2.41 gear ratio.
For my 80 (5th Gen) I got a 4.10 gearset from Richmond Gear. It was a 3.08:1 from Factory. So I purchased the gearset that fit a 3.08 and Numerically lower carrier. Unless I don't remember correctly that was the brake. 3.08 and lower or 3.08 and higher, I think is how they advertised them.
My 4.10 is there and works and I didn't have to fuss with getting a new carrier. I have to say, on looks alone, they are small rear ends. However they are not bad for street machines with most all small blocks. I think that if I was to put in a BBC then I'd try to go with the 8.5 out of a 4th or eariler or even a 12 bolt. I think that the 8.5s from the 4th gens are really strong and overall they are only, at best, a little smaller than the 12 which is something like 8.75".
Just for nothing, the Ford 9" absorbs more power than the GM 12 bolt. So the overall loss of torque from flywheel to rear wheels is more for the Ford 9" than the GM 12. For street machines that is not that big of a deal. For strip that is a tenth or so in the final ET.
Just thought that I'd toss that in.
And that 4:10 is behind a 700R4. I wouldn't do that large a gear without having an overdrive in front of it. For the 76 I will eventually go with an overdrive and taller gears, it has a 2.73:1 right now. However I have decided that upgrade will also be 4.10 and the tranny would be the 200R4 and not the 700R4. The gear ratio between 1 & 2 are better and the final drive is .63 where the final drive for the 700R4s are .70. So fuel economy is spared slightly more. And I'll use another PowerTrax Non-Slip in the diff. I like those. They are kind of wired to get use to but they have great potential.