For the purpose of paying the costs of the assessment portion of the work, the district shall issue its negotiable assessment paving bonds, referred to in section 39-1616, in an amount determined by deducting the special assessments paid within the period provided in section 39-1626 from the total assessment costs. The bonds shall mature in not more than twenty years as determined by the board of trustees of the district. The assessments plus interest collected after the fifty-day waiting period shall be deposited in an assessment bond sinking fund and, if for any reason, the assessments so collected are not sufficient to pay the assessment bonds when due, it shall be the mandatory duty of the board of trustees of the district to cause to be collected as a tax in the manner referred to in subsection (1) of section 39-1621 against all the property in the district sufficient to make good such deficiencies so that the assessment bonds shall be paid promptly at their respective due dates.