In multi-county senatorial districts, the amounts of assessments to be paid by candidates for the office of State Senator at the time and place of filing notwithstanding the provisions of § 7-11-410, shall be fixed by a majority of the county chairmen of the counties in the respective districts and shall be prorated among the county committees of the counties comprising the district in proportion to the number of precincts in each county. Provided, if such chairmen of any district fail to reach agreement within three days after the opening for entries the state executive committee shall fix the fee. Provided, further, that in 1966 only the chairmen shall have seven days in which to reach such agreement.