Here are a few courses from Texas and Oklahoma. I am not sure they all merit being in the Top 50 discussion but they are all pretty good courses under 6,500 yards from the back tees.
Brackenridge Park Golf Course - 6,263 yards (A.W. Tillinghast), San Antonio, TX
Dornick Hills - 6,453 yards (Perry Maxwell), Ardmore, OK
Onion Creek Club (Original Course) - 6,351 yards (Jimmy Demaret), Austin, TX
River Crest Country Club - 6,383 yards (Tom Bendelow), Fort Worth, TX
Stevens Park Golf Course - 6,285 yards (Jackie Burke Sr.) Dallas, TX