The finding of the board that the lands included within the general improvement district will be benefited by the proposed improvement or acquisition of property or rights therein, shall be a legislative determination that such lands will be specially benefited to the extent necessary to pay in full all costs and obligations of every nature required in making and maintaining such improvement or for the acquisition of property or rights, and such determination shall be conclusive upon the courts, except for actual fraud or arbitrary action on the part of the district board when making such finding as to lands benefited.