[GStage1]

Olds 12 bolt has a circular cover and a 10 bolt ring gear. Sounds like that is your rear axle. Chevy 12 has an oval cover and 12 bolt ring gear.

Olds 12 bolts are not worth your time, effort and $$$ to set-up. You are better off with the corporate 8.5" from a 71-2 Skylark/Cutlass...much better parts selection and cheaper prices since the center section, 8.5" ran thru 87 with 28 spline axles.