Way back then, the floor was attached to the chassis, at the front and rear only.
Then two hoists lifted that assembly off the ground and turned it 90 degrees, so that the floor and chassis were now vertical.
Then a workman would install the rest of the floor bolts.
Obviously. the weight of the chassis would cause it to droop, and was then locked into place when the remainder of the floor was attached to the frame.
Of course, the longer the frame, the greater the weight, the greater the droop.
Fortunately, the shell was later down the line, attached to the floor, which was in a straight line.
It took a while to correct that method of floor attachment.
Anyone that has visited the plant and went on "the tour" has seen a complete change in the way the shell and floor are now attached to the chassis.
The chassis, or frame, is held in a "jig" assuring a straight line.
Then the shell/floor assembly is lowered onto the chassis, and fastened in place.
Ah, how times have changed.