A mounting rail and gang box member installation system for placing a gang box on a wall structure in a desired horizontal and/or vertical location to accommodate for construction anomalies. The mounting rail and gang box member system includes an elongate mounting rail assembly and any one of an extendable box assembly, slidable bracket assembly or a rail channel structure that in combination with a gang box member and/or mud ring member provide for flexibility in locating and mounting wiring receptacles and wiring devices on a wall structure. Finally, the installation system includes wire or cable fastening configurations that are used in combination with cable ties or zip-ties to secure wires of cables without having to use staples.