I never had this same issue, but several years ago, my 74 F350 was drifting side to side, and had a fair bit of play in the steering, even though the kingpins and all the linkage was nice and snug. I talked to a local steering box rebuilder, and he told me these Ford power steering boxes had a habit of wearing the hole in the box for the sector shaft egg shaped over time. They had no bearing or bushing where the sector shaft went thru the bottom of the steering box casting, just a bored hole in the cast iron case. I looked underneath as somebody wiggled the steering wheel back and forth, and you could easily see the sector shaft just above the pitman arm move side to side. Not only did that play contribute to the slop in the steering, it also caused the lower seal to leak fluid, directly onto the left radius rod rubber bushing, causing the bushing to swell up and split. I had the steering box guy rebuilt the PS steering box, he also bores out the box casting and presses in a bearing to correct the worn bore during the rebuild process. Between the fresh steering box, and replacing the swelled up & split radius arm rubber bushings with some Energy Suspension PolyUrethane bushings, the improvement was amazing, it drove so much better afterwards.