31 U.S.C.§ 310, which is part of Title 31, Subtitle I, Chapter 3, Subtitle I (defines the organization of the U.S. Department of Treasury), makes FinCEN a bureau of Treasury.
A short note: Section 365 actually amended 31 U.S.C.§ 5324 to state the following:
(b) Domestic coin and currency transactions involving nonfinancial trades or businesses- No person shall, for the purpose of evading the report requirements of section 5333 or any regulation prescribed under such section--
(1) cause or attempt to cause a nonfinancial trade or business to fail to file a report required under section 5333 or any regulation prescribed under such section;
(2) cause or attempt to cause a nonfinancial trade or business to file a report required under section 5333 or any regulation prescribed under such section that contains a material omission or misstatement of fact; or
(3) structure or assist in structuring, or attempt to structure or assist in structuring, any transaction with 1 or more nonfinancial trades or businesses.