If any viaduct improvement district described in § 14-321-101, where it is or may be provided by law that the interest on bonds issued by the district shall not exceed six percent (6%) per annum, nor the bonds be sold for less than par, bonds bearing a lower rate of interest may be issued by the district and sold at less than par, provided that the proceeds of the sale of the bonds issued by the district shall not be less than the proceeds of the sale of six percent (6%) bonds at par.