Another option in GSMNP would be start at the Cosby campground lot, Take the Low Gap Tr south to the AT, Go NE to Mt Cammerer, take the Chestnut BR TR down to Big Creek, then take the Benton MacKaye Tr to Laurel Gap, then the Balsam Mtn Tr to Tricorner Knob on the AT, and then loop back to Cosby via the Snake Den Ridge Tr or Low Gap Tr. Total mileage 36-40. About 3 night, 4 days.
http://www.nps.gov/grsm/p...sit/upload/trails2005.pdf Doh! 50-75 miles is what you asked for. sorry.
I guess instead of heading north on the Balsam Mtn Tr, just head further SW then cut over to the AT on the Hughes Ridge Tr. That will add about 18M.