The notice of the referendum and the ballot to be used thereon shall contain a statement setting forth the changes or revisions to be made in the plan of benefit and contributions governing the retirement system, and the obligations which would be imposed on the members of the system if their services are included under an agreement pursuant to the provisions of this chapter. Total benefits to the present members, and to the dependents of those members, under the plan of coordination of the benefits of the retirement system with the old age and survivors insurance provisions of title II of the federal Social Security Act, 42 U.S.C. § 401 et seq., shall at least be equal to the benefits payable to them under the retirement system prior to social security coverage.