The three candidates for office of trustee who shall receive the highest number of votes shall be declared elected trustees with terms as provided in section 249.140. If it appears that the constitutionally required percentage of the voters of such district voting on the proposition of incurring indebtedness submitted at the election were in favor of incurring such indebtedness, the election authority shall make an order reciting the holding of such election and the results thereof, both for and against the proposition, and if the result of the election as certified shall be in favor of incurring the indebtedness and issuing the bonds, then the board of trustees for the district shall direct the issuance thereof to the amount of the debt authorized to be incurred, or any portion thereof, and shall either before or at the time of doing so provide for the collection of an annual ad valorem tax upon all of the taxable property within the district, which tax shall be sufficient to pay the interest on such indebtedness as it falls due and also to create a sinking fund for the payment of the principal thereof within twenty years from the date of contracting the same, such tax to be levied and collected as provided for in section 249.130.