The governing body may contract with respect to the safekeeping and application of the refunding bond proceeds and other funds included therewith and the income therefrom including the right to appoint a trustee which may be any trust company or state or national bank having powers of a trust company within or without the state of Washington. The governing body may provide in the refunding plan that until such moneys are required to redeem or retire the bonds to be refunded, the refunding bond proceeds and other funds, and the income therefrom shall be used to pay and secure the payment of the principal of and interest on the refunding bonds. The governing body may additionally pledge for the payment of revenue refunding bonds any revenues which might legally be pledged for the payment of revenue bonds of the issuer of the type to be refunded. Provisions must be made by the governing body for moneys sufficient in amount to accomplish the refunding as scheduled.