OK, here's the deal. The "nut up" VS "nut down" is the early (67-68--72) versus 73/ later steering setup, IE small spline boxes, pitman arm to fit, and center link and idler to go with that pitman. That whole "group" must reside as one, regardless of the year of the car.
You sure what spline size your boxes are? If they are both large, or both small, then you should be able to just swap the pitman from what is in the car, to the new box
THIS SUCKS!!! AFTER THE "IMPROVED SOFTWARE" I CAN NO LONGER FIND THE OLD POSTS
You can get part no's from the "tech" pages at BigBlockDart......
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