I have an 81 Cutlass Supreme and am looking to change the steering box I was wondering if the 4 bolt pattern and circular pattern steering boxes interchangeable?
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Either will bolt up to the frame the same way and to your steering column. But---

If you go with the 800 series box (4 bolt side cover) you will need to get the corresponding G-body pitman arm for an 800 series. Assuming your current one is the 600 series (round cover) you can simply take off the box along with popping the center link out of the pitman arm because the pitman arms are not interchangeable between the two types.

 

Thanks!!!!!