In order to secure funds for the making and completion of the improvements in the improvement district according to the changed plans, the board of improvement may borrow money not exceeding the full amount of the estimated cost of making and completing the improvements, with ten percent (10%) added for overhead cost, at a rate of interest not exceeding eight percent (8%) per annum, and may issue negotiable notes or bonds of the district for the payment thereof, and pledge all uncollected assessments for the security of the payment of the notes or bonds.