An apparatus and method are provided for assembling a roof or floor of a cargo body, and for securing doors and other portions of a cargo body in place. In some cases, a plurality of sidewalls are provided, each comprising a side panel and an upper rail and/or lower rail coupled to the side panel. A roof panel can be located above the plurality of sidewalls adjacent the upper rails, with a roof bow extending between the upper rails below the roof panel and fixed to a track of one of the upper rails with a fastener. A floor cross member can be located between the lower rails, and can be secured to the lower rails via a track and fastener.