This amendment shall be the sole authority required for the authorization, issuance, sale, execution and delivery of revenue bonds authorized hereby; provided, however, that the rate of interest on revenue bonds shall not exceed the maximum authorized by Amendment No. 60 to the Constitution of the State of Arkansas or any similar provision hereafter adopted. Nothing herein shall be construed to impair the initiative and referendum powers reserved to the people under Amendment No. 7 to the Constitution of the State of Arkansas.