This is a Sept 2, 1976 case from the US District Court, District of Columbia which gives an excellent discussion of the Five Tribes Act.
Civ. A. No. 74-189
Cite as 420 F.Supp. 1110 (1976)
This case is brought by four members of the Creek Nation for declaratory and injunctive relief; main issue was the DOI only dealing with the Creek Chief and refusing to recognize the Creek council;
page 1113 item 28 of this case states: "The Act of October 22, 1970, pertaining to selection of principal officers of the Five Civilized Tribes of Oklahoma, was intended to facilitate tribal self-determination and had no effect on the legal authority of the legislative branch of the Creek national government and under federal and Creek law the Creek national legislature retains the authority to make the initial decision controlling the expenditure of Creek funds for tribal purposes....federal officials acted illegally in recognizing the principal chief as the sole embodiment of the government in the Creek Nation."
Sorry I've not found a link to an online copy yet...this is a lower court opinion and they usually don't go back as far on the web as Supreme Court cases. This case also cites the Debo books quite heavily.