That "box" is the axle spacer. Pay attention to the orientation of the spacer which should not have more than 0.014" gap between the side of the spacer and end of axle. That spacer can be turned one quarter turn to either increase or decrease that gap, as the spacer is machined to different dimensions.

Roy Rodgers has it right but it might help to look at a service manual description of this process. Google "Keith Hardy Chevy Manuals" and look under the rear axle section.
kent


1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix