Actually there is much confusion regarding the authenticity of the available version of Seattle’s speech and a dearth of reference on the identity of the White Chief and Great Chief mentioned by him. But a discussion on The Free Dictionary Language Forum hints that Chief Seattle meant the Governor of Washington Territory Isaac Stevens as “White Chief” and the President of America as the “Great Chief“. And I think ‘Big Chief’ and ‘Great Chief’ refers to the same person, the president of America. Though the President at the time of the speech was Franklin Pierce, Seattle actually referred to the first President George Washington, as the native Americans thought that he was still alive. [This is confirmed by Henry Smith’s version published here]

The Good Father at Washington refers to George Washington. Chief Seattle mentions that since King George has moved his boundaries further north, he has become the father of the Native Americans.

The White Chief, Issac Stevens, sent the proposal of buying the Red Indians’ lands, as Henry Smith’s transcript of the speech claims.

[For detail reading on Chief Seattle, I recommend visiting this WIki entry.]