NEW YORK--(BUSINESS WIRE)--Fitch Ratings has affirmed the SLM Student Loan Trust 2004-6 and SLM Student Loan Trust 2004-7 senior and subordinate notes at 'AAAsf'. The Rating Outlook on the class B notes of both trusts has been revised to Negative from Stable. The Outlook revision is due to Fitch's belief that the class B notes of both trusts carry a heightened level of extension risk. Based on Fitch's cash flow modelling runs, the notes were not paid in full by their legal final maturity date o