I rebuilt my steering box in 2018. I had the EXACT same issue as you. When setting the preload on the bearing, I matched up my marks. What I didn't take into account is that the bearing needed to spin another two times before it was tight. I had at least 8 to 10 inches of play in the steering wheel. I could see input on the steering box before the pittman started to move. After spinning preload down, play was close to the OEM spec of 40mm for the dead zone.So two years later, I decided to get a new steering box head and had it installed. I will let you know if that helped.
BTW, opening a steering box is no fun and messy but I had to do it three times. On the third time and while still messy, I could take it off the truck, disassemble, rebuild, and reinstall in 2 hours.