Subject to the foregoing provisions of this article limiting the length of vehicles and loads, the load upon any vehicle operated alone or the load upon the front vehicle of a combination of vehicles shall not extend more than three feet beyond the foremost part of the vehicle and the load upon any vehicle operated alone or the load upon the rear vehicle of a combination of vehicles shall not extend more than six feet beyond the rear of the bed or body of such vehicle; provided, that the hydraulic boom and bucket permanently attached to a vehicle used in the maintenance and construction of electric service lines shall not be considered as load within the meaning of this section. Provided, further, that such boom and bucket shall not extend more than eight feet beyond the foremost part of the vehicle.