If an improvement district has been established in a city of the first class, an improvement thereon constructed, and curbing, or curbing and guttering, is therewith constructed and it becomes necessary to issue and sell street improvement bonds to pay for the cost of construction of the improvement and the curbing, or curbing and guttering, the mayor and city council may, at their discretion, include the cost of curbing, or curbing and guttering, with the cost of other improvements in the improvement district, and issue bonds for the combined cost of the improvement and curbing, or curbing and guttering, in any of the districts, naming the bonds Street Improvement Bonds of District No. …….. . The amount of money necessary for the payment of such bonds shall be levied upon and collected from abutting and adjacent property and property specially benefited as a special assessment.