I had a long thread on a similiar problem last year. Took forever to find, once I pulled all my hair out it was the pitman arm. 69 has a diffrent drag link set up that is hard to find as you know. I was never able to find a replacement pitman as it is a non wearable item on a 69 400. The teeth wear on the pitman arm and cause it to loosen, while remaining torqued to spec. The best way I can describe is it feels fine then all of a sudden it feels like you are driving on ice.

The steering box is a tapered fit so I got myself another lock washer and placed it between the pitman and steering box nut. (Two washers stacked). This allowed the pitman to drive further up on the gear box teeth and have a positive connection. Once I took it out for a test drive it was fixed and a world of difference. Hope this helps.