Since I'm here, I'll add that the frames were received as individual rails, crossmembers, etc and built up on the chassis line.
The operator would select the desired frame rails (1/2, 3/4 or 1 ton, 2wd vs 4wd, and wheelbase) and set them on a "pedestal line". The moving pedestals not only held the rails at a comfortable working height but also provided the fixturing to make sure the frame was square. The appropriate cross members were built up separately*, in sequence, and then installed as the rails moved on down the line.
I should also mention the frames started out upside down, to allow gravity to hold the front and rear suspension/axles in place until they were torqued down. During the process the frame was transferred from the pedestals to being hung from chains, and then flipped right side up midway through the process using a hydraulic chain hoist.
K