After the confirmation of the apportionments by the commission, the board shall cause to be prepared by the secretary of said board a special assessment roll assessing the estimated cost of the project (or if the actual cost has been ascertained, then such actual cost) against the several public corporations in accordance with the confirmed apportionments. The board may provide for the payment of such special assessments in any number of approximately equal annual installments, not exceeding 20. Installments of assessments against the state and against public corporations which collect their taxes beginning approximately December first in each year shall become due and payable on or before April first of each year. Installments of assessments against other public corporations shall become due and payable on or before such date or dates as shall be fixed by the board, depending upon the time or times of the collection of taxes by such public corporations. The board shall fix the amount of interest, not exceeding 6%, to be paid upon unpaid installments, which interest shall become due annually on the day and month upon which the annual installments become due. Any installment or installments may be paid in advance of the due date thereof with interest computed to the next installment due date. The board shall fix the time or times for the payment of the first installment so that each public corporation can make a tax levy for the payment thereof.