(b) Notwithstanding the provisions of Article III, section 29 subsequent to a Presidential declaration of an emergency or of a major disaster in any part of this Commonwealth, the General Assembly shall have the authority by a vote of two-thirds of all members elected to each House to make appropriations limited to moneys required for Federal emergency or major disaster relief. This subsection may apply retroactively to any Presidential declaration of an emergency or of a major disaster in 1976 or 1977.