Spending Limitation Act - Amends the Congressional Budget and Impoundment Control Act of 1974 to limit the total Federal outlays agreed to in the second or any further concurrent resolution on the Budget to the following percentages of the gross national product: 21 percent in fiscal year 1980; 20 percent in fiscal year 1981; 19 percent in fiscal year 1982; and 18 percent in the fiscal year 1983. Authorizes the Congress to waive such limitations by a two-thirds vote of each House.