NEW YORK--(BUSINESS WIRE)--Fitch Ratings has affirmed the SLM Student Loan Trust 2007-6 and SLM Student Loan Trust 2007-8 senior notes at 'AAAsf' and subordinate notes at 'Asf'. The Outlook on the class A-4 notes of both trusts has been revised to Negative from Stable. The Outlook revision is due to Fitch's belief that the class A-4 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 mat