Help with 70 Z28

[SS427]

I am very confused. Several years ago I restored Mark Mitchell's Classic Copper 70 Z28 which had factory installed manual steering. On that manual steering box is the pitman arm casting number 3964881 (as seen in this photo). The 5679142 box, arm and all steering components were original to the car.

Enter 2020 and a new Z28 restoration on a Cortez Silver RS/Z28 which came to me with power steering but per the buildsheet it should have manual. Great, so I have a correct 54679142 box and took the 3964881 that was mounted on the power steering box off to use on the manual box but was confused because I know that the pitman shaft is a larger diameter on the power steering cars than on the manual. Sure enough, the 3964881 pitman arm that I have will not fit on the manual box shaft but has the exact same part (casting) number. What gives?

I was on Nasty Z28 on an old thread and some claimed the 3964877 is the correct arm while others say that is not correct on the Z28 and it should use the 3964881 arm for the manual box. I fail to believe the GM would machine the pitman shaft opening for either a power steering car or a manual steering car without changing the part number but that is what I have and this arm will NOT fit (sloppy) on the manual pitman arm. Some of the guys on Nasty also said the 4881 arm is 7" long. Mine measures 8 5/8" overall and 6 7/8 center to center.

Can someone clear the air and does anyone else have a 3964881 arm on a manual steering box or one made for a manual box they would part with?
Thank you in advance for any help you can provide.
Rick