OPGI lists two quick ratio boxes, new and rebuilt, that fit 61-72 Skylarks but different size and spline shafts. One is 3/4 30 spline and the other is 13/16 36 spline. Did it vary by model at all? I have a 68 Custom 350-4 car. Haven't pulled it off yet, was hoping to to swap same time.
One is newer and the other is older style. I can't remember which is which but they make a rag joint that handles it. I put an '87 GN box in my 70 Stage 1 and it worked just fine. Also, the lines may be different as well. I had to use adapters to make the GN box work with my inverted flare lines.
Get the 87 GN box and an adapter set from lees power steering to run your current PS lines. You will need to get the rag joint referenced in the description below. https://leepowersteering.com/collec...ng-gearbox-seat-kit-after-1980-sae-conversion
