Whenever by the terms of the Project Act [43 U.S.C. 617 et seq.] or this subchapter payment of interest is provided for, and whenever interest shall enter into any computation thereunder, such interest shall be computed at the rate of 3 per centum per annum, compounded annually: Provided, That the respective rates of interest on appropriated funds advanced for the visitor facilities program, as described in section 619 (a) of this title, shall be determined by the Secretary of the Treasury, taking into consideration average market yields on outstanding marketable obligations of the United States with remaining periods to maturity comparable to the reimbursement period of the program during the month preceding the fiscal year in which the costs of the program are incurred. To the extent that more than one interest rate is determined pursuant to the preceding sentence, the Secretary of the Treasury shall establish for repayment purposes an interest rate at a weighted average of the rates so determined.