You are correct, the torsion bars (@ least back in the GMT400 days) were set a about 15 or 20 minutes before gas fill. We were building 60 jobs per hour, so each operator has a minute to do their job on each truck.
That being said, I think it would be possible to fill a truck off line & then adjust the bars so that tge truck was level. Take what was found and apply it to the specs for adjustment. From my memory and we're going back to 1988, the adjustment fixture was symmetrical from side to side.
As Ive said elsewhere, I dont recall seeing a similar fixture being used on 'the flat top' conveyor during the GMT800 or GMT900 model runs. Maybe it was adjusted in the Chassis Area after the front suspension is fully assembled? I didnt have much to do w/ that area, so I cant say anything certain about the later assembly procedures.