A propulsion unit for an aircraft including a nacelle with a D-shaped structure housing a thrust-reversing device with movable vanes, the nacelle containing two D-shaped half-structures each including an external reverser half-cowl. The propulsion unit contains an assembly box-type structure, attached in downstream cantilevered fashion to the turbojet engine fan casing, the box-type structure including two guide rails guiding the deflection vanes, a locking device between the assembly box-type structure and the half-beams in the six o'clock position of the D-shaped half-structures, the assembly box-type structure being arranged in the propulsion unit such that the guide rails guiding the vanes of the assembly box-type structure are situated in the continuation of the guide rails in the six o'clock position guiding the vanes attached to the fan casing, to provide continuity between the rails.